Add a request for setting response prefix

[vlad20012]

Currently, if a proc macro prints something to the stdout (println!("stuff {{}}")) the output mixes with the proc-macro-srv protocol messages (proc-macro-srv uses stdout to communicate with rust-analyzer) and rust-analyzer then can't distinguish proc-macro-srv JSON messages from the output from the proc macro.

My idea is to prepend proc-macro-srv protocol messages with some long string, and then skip the proc-macro-srv process output in rust-analyzer until that known string (a bit similar to how Ethernet framing works). I.e. the proc-macro-srv message should look like "OsdgHOghs895whrhg598{response_fields: {...}}"

I don't want to hardcode some specific string because it could be exploited by some macro, or that string could become a problem if we decide to place that string inside a macro in rust-analyzer itself. So I think we should generate a random string.

I added a new requst type msg::Request::SetExpanderSettings with response_prefix field in order to delegate random string generation to rust-analyzer (and keep the proc-macro-srv implementation as simple as possible).

In this PR I just change the proc-macro-srv part