[analysis_server] Refactor/rename server to simplify sharing handlers between LSP+Legacy protocols

In order to share LSP handlers between protocols, we need to be able to use a LegacyAnalysisServer in place of an LspAnalysisServer which means having a common interface (which they have via AnalysisServer) and to make the mapping of LSP method names to handlers reusable.

This changes makes the following non-functional changes (to reduce the size of a future change that will begin sharing handlers):

- Rename "clientCapabilities" to "lspClientCapabilities" to avoid conflicts with legacy servers clientCapabilities field when we add this to the base AnalysisServer interface
- Rename "clientConfiguration" to "lspClientConfiguration" for consistency
- Add an "LspMessageHandler" typedef over "MessageHandler" which now has a server type arg so that it can be used with either LspAnalysisServer (by handlers that really need LSP) and AnalysisServer (for handlers that can work in either server)
- Extract handler generators in `InitializedStateMessageHandler` to a static map and make constructors consistent taking only a server (in a future CL the legacy server will want access to some this mapping)

Change-Id: I23554a7ca318fbcd1113dcebff5601186b223618
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/311982
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>