Add FirebaseAIConfig support

[daymxn]

Per b/430576238,

This adds support for the FirebaseAIConfig struct, alongside the AppCheckOptions struct. The limitedUseTokens aspect is implemented, and has been documented as well.

Limited-use tokens have a shorter TTL than the standard app check tokens, which helps provide a smaller attack surface for malicious attackers. Limited-use tokens are also required to take advantage of replay protection, which Firebase AI Logic will support in the future. Since limited-use tokens are backwards compatible, we can add support on the client SDKs before the backend supports replay protection; this will allow consumers to adopt limited-use tokens early, and enable replay protection right away when it releases.

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[ncooke3]

Great doc comments! LGTM on green CI and resolution of error conversation.

Additionally, please add a very brief comment to the PR description about why limited tokens are added. This can be reused in changelog entry.

For a changelog entry, since the version is not finalized, # Unreleased can be used as the section header.

[ncooke3]

Do you know how this error gets wrapped by Firebase AI Logic? NSError is mainly for an Objective-C context. Is it easy to get the error message out if your at the top-level trying to parse the internal error?

cc: @andrewheard