Commit Message: gcp_authn: simplify crypto utils
Additional Description:

Previously my understanding was that the gcp authn filter was going to have its own sds config to specify a certificate for generating bound tokens. However, due to incompatibility with grpc, this design was scrapped in favor of just directly the cluster's client certificate.

This makes the job of the crypto util simpler since it no longer needs to verify the cert. Instead it trusts that the cluster is configured correctly (otherwise the connection will likely fail anyway--so the token wont matter), so all it needs to do is generate a fingerprint. The san matching functionality has been removed.

Risk Level: none (changing unused code)
Testing: unit tests updated
Docs Changes: none needed
Release Notes: none needed

I used generative AI to create this change.

Request variables

Key	Value
ref	1657813
sha	c6a8cd4
pr	45385
base-sha	86c2632
actor	@antoniovleonti
message	gcp_authn: simplify crypto utils...
started	1780417020.021042
target-branch	main
trusted	false

Container image/s (as used in this CI run)

Envoy version (as used in this CI run)