INFOPLAT-1562 dynamic expiring auth headers

[hendoxc]

INFOPLAT-1560

What

Allows usage of dynamic auth headers by implementing grpc.PerRPCCredentials & setting as a DialOption on the otel client

Why

Makes tokens expire, in the case that one is leaked or intercepted.

• someone can indeed steal a token, update the timestamp portion of the token, then try send data to the gateway endpoint, however since pubkey bytes + timestamp bytes are what's being signed, the signature part of the token is invalid, and will be rejected by the gateway
• on the gateway, version 2 tokens have their timestamp part checked time.Now > timestamp > time.Now - serverTTL

Notes

Current users of the client can still configure static headers using the AuthHeaders field of beholder client Config. To enable dynamic headers they instead should configure AuthHeaderProvider