…Google vertex (#63282)

[Linear
Issue](https://linear.app/sourcegraph/project/claude-3-on-gcp-8c014e1a3506/overview)

This PR adds support for anthropic models in the google provider through
google vertex.
NOTE: The current code only supported Google Gemini API and had boiler
plate code for Google vertex(only for the gemini model) this PR adds
Google Vertex for anthropic models properly so this way the google
provider can be run in 3 different configurations
1. Google Gemini API(this works but only for chat and not for
completions which is the intended behaviour for now)
2. Google Vertex API Anthropic Model(This works perfectly and is added
in this PR and tested for both chat and completions and it works great)
3. Google Vertex API Gemini Model (this doesn't work yet and can
eventually be added to this codebase but we gotta add a new decoder for
the streaming responses of the gemini model through this API we can take
care of this later)

Sense of Urgency: This is a P0 because of enterprise requirements so I
would appreciate a fast approval and merging.

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- Run this branch for Cody instance ->
https://github.com/sourcegraph/cody/pull/4606
- Ask @arafatkatze to dm you the siteadmin config to make things work
- Check the logs and play with completions and chat

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---------

Signed-off-by: Stephen Gutekanst <stephen@sourcegraph.com>
Co-authored-by: Beatrix <beatrix@sourcegraph.com>
Co-authored-by: Stephen Gutekanst <stephen@sourcegraph.com>
(cherry picked from commit 1a6a7f7)

…Google vertex (#63282)

[Linear
Issue](https://linear.app/sourcegraph/project/claude-3-on-gcp-8c014e1a3506/overview)

This PR adds support for anthropic models in the google provider through
google vertex.
NOTE: The current code only supported Google Gemini API and had boiler
plate code for Google vertex(only for the gemini model) this PR adds
Google Vertex for anthropic models properly so this way the google
provider can be run in 3 different configurations
1. Google Gemini API(this works but only for chat and not for
completions which is the intended behaviour for now)
2. Google Vertex API Anthropic Model(This works perfectly and is added
in this PR and tested for both chat and completions and it works great)
3. Google Vertex API Gemini Model (this doesn't work yet and can
eventually be added to this codebase but we gotta add a new decoder for
the streaming responses of the gemini model through this API we can take
care of this later)

Sense of Urgency: This is a P0 because of enterprise requirements so I
would appreciate a fast approval and merging.

<!-- 💡 To write a useful PR description, make sure that your description
covers:
- WHAT this PR is changing:
    - How was it PREVIOUSLY.
    - How it will be from NOW on.
- WHY this PR is needed.
- CONTEXT, i.e. to which initiative, project or RFC it belongs.

The structure of the description doesn't matter as much as covering
these points, so use
your best judgement based on your context.
Learn how to write good pull request description:
https://www.notion.so/sourcegraph/Write-a-good-pull-request-description-610a7fd3e613496eb76f450db5a49b6e?pvs=4
-->

- Run this branch for Cody instance ->
https://github.com/sourcegraph/cody/pull/4606
- Ask @arafatkatze to dm you the siteadmin config to make things work
- Check the logs and play with completions and chat

<!-- All pull requests REQUIRE a test plan:
https://docs-legacy.sourcegraph.com/dev/background-information/testing_principles
-->

<!--
1. Ensure your pull request title is formatted as: $type($domain): $what
3. Add bullet list items for each additional detail you want to cover
(see example below)
4. You can edit this after the pull request was merged, as long as
release shipping it hasn't been promoted to the public.
5. For more information, please see this how-to
https://www.notion.so/sourcegraph/Writing-a-changelog-entry-dd997f411d524caabf0d8d38a24a878c?

Audience: TS/CSE > Customers > Teammates (in that order).

Cheat sheet: $type = chore|fix|feat $domain:
source|search|ci|release|plg|cody|local|...
-->

<!--
Example:

Title: fix(search): parse quotes with the appropriate context
Changelog section:

- When a quote is used with regexp pattern type, then ...
- Refactored underlying code.
-->

---------

Signed-off-by: Stephen Gutekanst <stephen@sourcegraph.com>
Co-authored-by: Beatrix <beatrix@sourcegraph.com>
Co-authored-by: Stephen Gutekanst <stephen@sourcegraph.com>
(cherry picked from commit 1a6a7f7)