Jsonrpc: add new jsonrpc-autoport-bootstrap helper

Jsonrpc: add new jsonrpc-autoport-bootstrap helper

This will help Eglot and some other extensions connect to network
servers that are started with a call to a local program.

* lisp/jsonrpc.el (jsonrpc--process-sentinel): Also delete inferior.
(jsonrpc-process-connection): Add -autoport-inferior slot.
(initialize-instance jsonrpc-process-connection): Check
process-creating function arity.  Use jsonrpc-forwarding-buffer
(jsonrpc-autoport-bootstrap): New helper.
(Version): Bump to 1.0.20.

(cherry picked from commit 9e24cde227a1bf2e1f0c005ca16b2a70e704ff5c)