## Summary

Add a `buildFavor` property to `Env` (accessible from the plugin's
initializer context), Mimicking the idea of ES's `version.buildFlavor`
field.

Note: this is not supposed to be a replacement for feature flags, but
can be useful when wanting to toggle features based on actual
capabilities of our serverless product. Also, we already expose this
value through the configuration via the `serverless` context value, so
it now adds another way to access the information.

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Co-authored-by: Kibana Machine <42973632+kibanamachine@users.noreply.github.com>

Closes #161337

## Summary

Uses build flavor(see #161930) to disable specific Kibana security,
spaces, and encrypted saved objects HTTP API routes in serverless (see
details in #161337). HTTP APIs that will be public in serverless have
been handled in #162523.

**IMPORTANT: This PR leaves login, user, and role routes enabled. The
primary reason for this is due to several testing mechanisms that rely
on basic authentication and custom roles (UI, Cypress). These tests will
be modified to use SAML authentication and serverless roles in the
immediate future. Once this occurs, we will disable these routes.**

### Testing
This PR also implements testing API access in serverless.
- The testing strategy for disabled routes in serverless is to verify a
`404 not found `response.
- The testing strategy for internal access routes in serverless is to
verify that without the internal request header
(`x-elastic-internal-origin`), a `400 bad request response` is received,
then verify that with the internal request header, a `200 ok response`
is received.
- The strategy for public routes in serverless is to verify a `200 ok`
or `203 redirect` is received.

~~blocked by #161930~~
~~blocked by #162149 for test implementation~~

---------

Co-authored-by: kibanamachine <42973632+kibanamachine@users.noreply.github.com>
Co-authored-by: Aleh Zasypkin <aleh.zasypkin@gmail.com>
Co-authored-by: Aleh Zasypkin <aleh.zasypkin@elastic.co>