[lldb-dap] Migrate attach to typed RequestHandler. (#137911)
This updates the `attach` request to the typed
`RequestHandler<protocol::AttachRequestArguments,
protocol::AttachResponse>`.
Added a few more overlapping configurations to
`lldb_dap::protocol::Configuration` that are shared between launching
and attaching.
There may be some additional code we could clean-up that is no longer
referenced now that this has migrated to use well defined types.