Add Request to Sendable

[nuid64]

Sendable represents types of DAP messages that can be sent via JSON RPC, so it would be nice if we add Request in here, since dap-rs supports client-side code.

[nuid64]

The problem is that Sendable derives Serialize by default and Deserialize if client feature is on, and Request does vice versa. Either Request (and thus every other type) must derive 'Serialize' by default or we must include Request variant in client feature, which looks pretty confusing because essentially Request is a sendable message