Update API to make headers available so we can provide an `external` option to search settings

[chipsenkbeil]

Could be a boolean like external = true to let the implementation pick an external process to execute.

Or could be a string like external = "ripgrep" to specify the external process to use.

Or is this needed? distant-ssh2 has to use an external process. distant-local uses an internal version of ripgrep.

[chipsenkbeil]

Do we have to include options? Or can we just enable the header for a request to be available as part of the api? The header route would enable us to add features like this without breaking protocol, so I'm going to move 1.0 milestone while this is considered.