[analysis_server] Create a wrapper to allow LSP helpers to be used over DTD

A minor refactor towards supporting CodeActions over DTD.

The (existing) `LspRequestHelpersMixin` is a collection of helper methods like `getHover()` that allow sharing strongly-typed access to LSP methods across different kinds of tests (LSP, LSP-over-Legacy, and in-process vs out-of-process).

This adds a class (`DtdHelper`) that allows those same helpers to be used to call LSP over DTD (it has to be a separate instance because these helpers already exist on the base test classes to communicate with the server directly without DTD).

As part of this, the mixin was split up to separate request, reverse-requests and notifications, since only outbound requests apply over DTD and it cannot provide an implementation for the others.

This change also adds `textDocument/hover` to the allow-list so that some of these tests can be unskipped (since in an upcoming change the allow-list will be removed entirely anyway).

Change-Id: Id7bf40020cb5397e4cb8bbdefeed71745f58c4cc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/428520
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>