Add Google client initialisation helper method to generate ID tokens

[deadlycoconuts]

Context

With the move to use OIDC identity (ID) tokens instead of OAuth2.0 access tokens to authenticate all users of CaraML (including end-users and more importantly in the context of this PR, other CaraML components), there is a need to update the way in which each individual CaraML component authenticates itself and the token which gets injected into the header of every single outgoing request from them.

As of present each outgoing request from a CaraML component has an OAuth2.0 access token appended to its header, and this process is handled by an HTTP client generated by one of the helper methods from the Google client library, not unlike the one below:

import (
	"golang.org/x/oauth2/google"
)

googleClient, err := google.DefaultClient(context.Background(), "https://www.googleapis.com/auth/userinfo.email")

However, this helper method does not generate an HTTP client that will append ID tokens to outgoing requests; a separate helper method from a different Google client library is needed to do that:

import (
	"google.golang.org/api/idtoken"
)

googleClient, err := idtoken.NewClient(ctx, audience)

To complicate things further, the Google client libraries does not (easily) allow ID tokens to be generated when a service account is not used for authentication, and instead defaults to generating access tokens. Hence, a separate method to generate the HTTP client is needed for scenarios when the other forms of credentials are used for authentication, such as most notably, user accounts.

This PR thus introduces a helper method that handles not only the HTTP client creation for authentication with a service account, but also for other types of credentials. The latter is done by manually initialising the token source of the credentials, and then manually extracting the ID token from them before encapsulating them in a regular HTTP client struct - this process is highly inspired by what was suggested as a hack by unhappy users here to retrieve the ID token manually when not using service accounts, as well as the Google client library's way of wrapping a TokenSource struct into an HTTP client.

This helper method will subsequently be imported and used by the various CaraML components to initialise an authenticated Google client, which will ensure that all outgoing requests from them would contain ID tokens, regardless of the type of credentials used.

[krithika369]

@deadlycoconuts, thanks for the detailed PR description and the neat solution. Left some small comments. The rest LGTM!

[deadlycoconuts]

Thanks a lot for the comments! I'll be merging this now! 🚀