Update grpc.rs

[HarshMN2345]

Summary:
This pull request introduces validation logic to handle repeated message types in gRPC schemas, treating them as optional during input schema validation. The updates ensure proper handling of complex message structures while maintaining schema consistency..

Issue Reference(s):
Fixes tailcallhq#2720
/claim tailcallhq#2720

Build & Testing:

• I ran cargo test successfully.
• I have run ./lint.sh --mode=fix to fix all linting issues raised by ./lint.sh --mode=check.

Checklist:

• I have added relevant unit & integration tests.
• I have updated the documentation accordingly.
• I have performed a self-review of my code.
• PR follows the naming convention of <type>(<optional scope>): <title>