### Motivation and Context

<!-- Thank you for your contribution to the semantic-kernel repo!
Please help reviewers and future users, providing the following
information:
  1. Why is this change required?
  2. What problem does it solve?
  3. What scenario does it contribute to?
  4. If it fixes an open issue, please link to the issue here.
-->
As we are preparing to move towards v1 where the context no longer is
used, we need a better way of handling the responses from models and
capture them in a more structured way, also in preparation of
multi-modal models and the like.

Closes microsoft#4598 

### Description

<!-- Describe your changes, the overall approach, the underlying design.
These notes will help understanding how your code works. Thanks! -->
Added KernelContent and StreamingKernelContent base classs, consistent
with the dotnet versions of those.
Subclasses of this class for specific services, for instance, the OpenAI
class has parsing for the content, function_calls, tool_calls in it, and
the Azure OpenAI one has parsing for the context in it.

### Contribution Checklist

<!-- Before submitting this PR, please make sure: -->

- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [ ] I didn't break anyone 😄