Feature: Add support for grpc middleware interceptors

[cmmoran]

📝 Checklist

• All new/changed/fixed functionality is covered by tests (or N/A)
• I have added documentation for all new/changed functionality (or N/A)

🔧 Changes

• Expanded the TokenExtractor by creating a ContextTokenExtractor to allow extracting a token from metadata.
• Expanded ErrorHandlers by adding grpc unary and stream default error handlers that loosely mimic the same functionality as the http-based default error handler
• Expanded JWTMiddleware by creating GrpcMiddleware which contains a unary interceptor and a stream interceptor that can be optionally specified via the new GrpcOption type
• Omitted validateOnOptions for GrpcOption as it didn't seem relevant
• Included credentialsOptional for GrpcOption to allow optional credentials for grpc (as it seemed relevant)
• Added a trivial example under the examples folder: grpc-example for relevant grpc usage.

📚 References

#181

🔬 Testing

• I tried to keep true to the ContextKey{} paradigm so that the server-side handler can check the context for this key to contain the *validator.ValidatedClaims structure for validated token claims
• ErrJWTInvalid and ErrJWTMissing are utilized through the grpc status.Errorf function to return the relevant status with the error

[agruetz]

Any ideas when this might get merged? This is exactly what I need. Is there anything I could do to help move it forward quickly?