We agreed with the build team to move streams specific dependencies into the mongo/src/third_party/enterprise directory.

• We will move librdkafka and mongocxx into this directory. Future streams-only dependencies will also live here.
• This directory will be excluded from the open source repo.
• We will discuss with build team whether a "streams specific" directory i.e. mongo/src/third_party/enterprise/streams is worthwhile.