debugcommands: support sending HTTP requests with debugwireproto We implement an action that can issue an HTTP request. We can define headers via arguments and specify a file to use for the HTTP request body. The request uses the HTTP peer's opener, which is already configured for auth, etc. This is both good and bad. Good in that we get some nice behavior out of the box. Bad in that some HTTP request headers are added automatically. Differential Revision: https://phab.mercurial-scm.org/D2841