feat: Dynamic GRPC Metadata

[ekawinataa]

Criteria :

• Enable dynamic gRPC metadata based on payload
• Payload is parsed using CEL expression and mapped to simple key value header
• Metadata could contains both static and dynamic key-value
• Using CEL Expression to parse the Proto payload, for details please refer https://github.com/google/cel-spec/blob/master/doc/langdef.md. The reason of using CEL expression is because it natively support proto/DynamicMessage without the need to parse it to JSON first
• Should use full qualified proto class name for the CEL Expression
• Only supports primitive value as the key and value for the metadata

Reason for change :

• Some of EGLC users that want to migrate are expecting Firehose to have the feature to insert gRPC metadata based on the payload/event

Configuration would be something like this :

SINK_GRPC_METADATA=staticKey : $Payload.field_one,Payload.field_two: $Payload.fields[0].name