[Feature] Support extensions for proto2

[zhihuizhang17]

Summary

This PR implements extension support for proto2 by:

• Leveraging Protobuf.Extension.get_extension_props_by_tag to query extensions by type and index
• Adding convert_to_field_descriptor to convert extension props to field descriptors
• Implementing process_extensions to generate file descriptors for extensions
• Storing xxxExtension.proto file descriptors in Agent.files
• For :all_extension_numbers_of_type request, tracking type extension field numbers in Agent.extensions, such as

  %Agent{
  "extensions": {
  "helloworld.HelloReply": [100, 102, 502]
  },
  "files": {
  "helloworld.HelloReplyExtension.proto":  <<binary>>,
  "helloworld.HelloReply.proto":  <<binary>>
  }
  }

Unit test

Test cases run with proto2.

• test :file_containing_symbol

  
  > grpcurl -plaintext localhost:50051 describe .helloworld.HelloReply  

helloworld.HelloReply is a message: message HelloReply { optional string message = 1; optional .google.protobuf.Timestamp today = 2; extensions 100 to 199, 500 to 1000; }

+ test `:all_extension_numbers_of_type` and `file_containing_extension`

grpcurl -plaintext -format text -d 'name: "gRPCurl"' localhost:50051 helloworld.Greeter.SayHello

message: "Hello gRPCurl" today: < seconds:1702870608 nanos:991037583

[helloworld.extend_message]: "extended message"

id: 1 name: "name"