Feat: Firestore support

[ScottMansfield]

This is a draft PR for sharing. Will update and fill in later.

This copies the Firestore support from the Java ADK into the Python ADK.

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[adk-bot]

Response from ADK Triaging Agent

Hello @ScottMansfield, thank you for creating this PR!

I see that this is a draft PR. When you are ready, please ensure you have read the contribution guide and filled out the PR template, including:

• Link to an existing issue or a description of the change: This helps reviewers understand the context of your contribution.
• Testing Plan: Please describe the tests you ran to verify your changes.

This information will help reviewers to review your PR more efficiently. Thanks!

[ScottMansfield]

Looks like we have one open for the community repo: google/adk-python-community#104

[rohityan]

Hi @ScottMansfield , Thank you for your contribution! We appreciate you taking the time to submit this pull request.
Closing this PR here as it belongs to adk-python-community repo.
We highly recommend releasing the feature as a standalone package that we will then share through: https://google.github.io/adk-docs/integrations/