Relay Proxy: support GCP IAM authentication for Memorystore

[msvorad]

Is your feature request related to a problem? Please describe.
GCP counterpart to #680, but with a sharper constraint: on Memorystore for Valkey,
IAM authentication is the only generally available authentication mechanism.

Describe the solution you'd like
A GCP token provider alongside the AWS one being prototyped in #691.

Describe alternatives you've considered
GCP supports only IAM_AUTH or NOAUTH, static tokens are in preview mode only.

Additional context
SDK needs to support IAM auth as well.

[aaron-zeisler]

Hi @msvorad and thank you very much for opening this issue. I see that Memorystore for Valkey makes IAM effectively mandatory (IAM_AUTH or NOAUTH only; basic-auth tokens are preview), which is a different situation from AWS, where static passwords remain a supported fallback.

This work fits into the larger project that we're planning around modernizing Redis in the relay proxy (part of which is prototyped in #691). I'm hoping to work on this as part of our Relay Proxy v9 release. I'll report back when we have a better idea of the timeline.

We're internally tracking this as SDK-2462