Accept functions as entries in eglot-server-programs

Accept functions as entries in eglot-server-programs

CONTACT in the (MAJOR-MODE . CONTACT) association in
eglot-server-programs can now be a function of no arguments producing
any value previously valid for contact.  The function is called at
time of `M-x eglot` or `eglot-ensure`.  This is useful for servers
requiring command-line invocations that depend on the specific
momentary environment.

* eglot.el (eglot-server-programs): CONTACT can be a fucntion of no
arguments.
(eglot--guess-contact, eglot--connect): Accept function
CONTACTs.

GitHub-reference: per https://github.com/joaotavora/eglot/issues/63