Add validating interceptor

[elliotmjackson]

Relates to #2

This PR introduces an interceptor for Connect to validate incoming protobuf messages using protovalidate-go. Supports both unary and streaming RPCs.

📝 Default Behaviors:

• Requests are validated for adherence to the defined message structure.
• Responses remain unvalidated; the focus is on incoming data validation.
• Errors are raised for messages that aren't protocol buffer messages.
• In cases of validation errors, an error detail type is attached containing the Violations to offer more context about the failure.

🧪 Testing:

1. Test Scenarios:
   • Check that an empty request correctly reports required fields.
   • Validate that invalid values trigger the expected validation errors.
   • Ensure unrelated server errors (like internal server errors) remain unaffected by validation logic.
   • Confirm valid requests proceed without any validation issues.
2. Mock Server Utilization: Utilizes the testserver package to create in-memory servers for testing client-server interactions without the need for actual networking.

❓ Open Questions:

• Tests contain a bit of duplicated code(at least enough to raise suspicion of the linter) - I've chosen to ignore this warning in favor of readability. I'm open to reducing this duplication if its desired.
• More on linting: the exhaustruct rule is very annoying, particularly when dealing with functional option pattern as this requires an initialized and empty struct to be defined. I have turned it off but I feel we have a reason to be using it so I'm open to hearing what this reason may be.

[elliotmjackson]

I did this to perform a user-centric test strategy. This is not a pattern I usually employ but I see it throughout the connectrpc org

[elliotmjackson]

I pinched this from connect-go. Have we considered a connecttest sdk for users?