config: Make gRPC xDS retry backoff configurable (#24701)

The gRPC xDS implementation uses hardcoded retry durations for re-establishing the gRPC channel. This change makes the backoff parameters configurable.

Risk Level: Low
Testing: unit test

Fixes #24236

Signed-off-by: Jayson Dmello <jdmello@confluent.io>

Mirrored from https://github.com/envoyproxy/envoy @ 672b41532a9f03bc84fbbfec0d1db5b633833f9c

envoy/config/core/v3/grpc_service.proto

@@ -43,6 +43,12 @@ message GrpcService {
     string authority = 2
         [(validate.rules).string =
              {min_len: 0 max_bytes: 16384 well_known_regex: HTTP_HEADER_VALUE strict: false}];
+
+    // Indicates the retry policy for re-establishing the gRPC stream
+    // This field is optional. If max interval is not provided, it will be set to ten times the provided base interval.
+    // Currently only supported for xDS gRPC streams.
+    // If not set, xDS gRPC streams default base interval:500ms, maximum interval:30s will be applied.
+    RetryPolicy retry_policy = 3;
   }
 
   // [#next-free-field: 9]