[mlir-lsp] Support outgoing requests (#90078)

Add support for outgoing requests to `lsp::MessageHandler`. Much like
`MessageHandler::outgoingNotification`, this allows for the message
handler to send outgoing messages via its JSON transport, but in this
case, those messages are requests, not notifications.

Requests receive responses (also referred to as "replies" in
`MLIRLspServerSupportLib`). These were previously unsupported, and
`lsp::MessageHandler` would log an error each time it processed a JSON
message that appeared to be a response (something with an "id" field,
but no "method" field). However, the `outgoingRequest` method now
handles response callbacks: an outgoing request with a given ID is set
up such that a callback function is invoked when a response with that ID
is received.