agent/reconnectingpty: allow specifying backend type cli: exp rpty: automatically select backend based on command
Relates to https://github.com/coder/coder/issues/16419 Builds upon https://github.com/coder/coder/pull/16638 and adds a command `exp rpty` that allows you to open a ReconnectingPTY session to an agent. This ultimately allows us to add an integration-style CLI test to verify the functionality added in #16638 .
