Add lisp/external-completion.el for completions from external tools

This table uses a new 'external' completion style which has been in
use in Eglot and other third-party extensions for a long time.  It's
meant for libraries that interface with an external tool which
provides completions, such as a shell utility, an inferior process, an
http server.

The new file lisp/external-completion.el provides a helper function,
external-completion-table which is given an interfacing function to
the external tool and returns a "programmed completion" table that is
bound to the 'external' completion style.  That table can then be used
with completing-read or any other facility expecting a completion
table.

When the 'external' is in use, the usual styles configured by the user
or other in 'completion-styles' are completely overriden.  This
relatively minor inconvenience is the price to pay for responsive
completion where the full set of completion candidates doesn't need to
be transfered into Emacs's address space.

* lisp/external-completion.el: New file.

* etc/NEWS: (Lisp Changes in Emacs 29.1): Mention
  external-completion-table.

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