[Functions] Add support for streamable cloud functions

[ncooke3]

go/cf3-callable-streaming-clients-ios

Android PR: firebase/firebase-android-sdk#6602

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Contribution Workflow

1. Fork the Firebase repo and clone it.
2. Add Upstream: git remote add upstream https://github.com/firebase/firebase-ios-sdk.git
3. Create Branch: git fetch upstream && git checkout -b <feature-branch> upstream/streamable-functions
   • Where <feature-branch> is the name of the branch (e.g., add-tests-streamable).
4. Regular workflow: Run style script and git add . && git commit -m "..."
5. Push: git push origin <feature-branch>
6. PR: Create a PR on GitHub, with base branch set tofirebase:streamable-functions
7. Update fork feature branch (if needed):

git fetch upstream
git checkout <feature-branch>
git merge upstream/streamable-functions
git push origin <feature-branch>

[andrewheard]

Note: Just sending a few comments along before I head out. (Didn't see anything that didn't LGTM so far, just haven't finished reviewing yet)

[ncooke3]

/gemini please review

[gemini-code-assist]

Okay, I will review the pull request now.

[ncooke3]

/gemini Still reviewing?

[gemini-code-assist]

It appears I am still reviewing the pull request. I will provide feedback as soon as possible.

[ncooke3]

This should be in a good state. I'll address any further comments in a follow-up PR.