[analysis_server] Provide LSP ClientCapabilities to LSP handlers reflecting the calling client instead of always the server

Prior to this change, all LSP handlers would ask the server for the client capabilities when they would affect their output. For callers over DTD, this could result in the same requests getting different results depending on the capabilities of the editor.

This change adds LspClientCapabilities to the MessageInfo that goes along with each message (usually a request) and then passes in the servers clientCapabilities for requests originating from the editor client, and a fixed set of capabilities for requests that come from DTD.

Most handlers will now use the callers capabilities, however there are a few handlers that build edits to send to the editor (instead of _returning_ the edits), which will use the editors capabilities.

This is essentially a no-op for now, because we're just providing the same capabilities via a different route - but when we come to supporting LSP handlers over DTD (that are affected by client capabilities), they will get consisted results based on our fixed set of capabilities (which as of yet is probably incomplete and may be updated a little as that progresses).

Change-Id: I5bcef8c3ebb7b94c83d85e56a303333d302abdee
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/382881
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Keerti Parthasarathy <keertip@google.com>
Commit-Queue: Keerti Parthasarathy <keertip@google.com>