Begin adding extension numbers to SourceCodeInfo and FileDescriptorSet for tooling purposes.

Allocate extension number 536000000 to Buf

Extension numbers will be allocated one at a time by request.
Once extension declarations are open-sourced for extensions in descriptor.proto, we will move these declarations into a separate config file to declutter descriptor.proto.

PiperOrigin-RevId: 692315472

Author: protobuf-github-bot