add fbcode_builder_config for mvfst build in oss
add fbcode_builder_config for mvfst build in oss

Summary: support for fbcode_build and travis build

Reviewed By: siyengar

Differential Revision: D15184766

fbshipit-source-id: 5117164c324d3d3c867592a88670a53b6f862477

rsocket/rsocket-cpp,ReactiveSocket/reactivesocket-cpp,phoad/rsocket-cpp,ReactiveSocket/reactivesocket-cpp,rsocket/rsocket-cpp,rsocket/rsocket-cpp,rsocket/rsocket-cpp,phoad/rsocket-cpp,phoad/rsocket-cpp,ReactiveSocket/reactivesocket-cpp,phoad/rsocket-cpp,phoad/rsocket-cpp