promisor-remote: allow a client to check fields

promisor-remote: allow a client to check fields

A previous commit allowed a server to pass additional fields through
the "promisor-remote" protocol capability after the "name" and "url"
fields, specifically the "partialCloneFilter" and "token" fields.

Let's make it possible for a client to check if these fields match
what it expects before accepting a promisor remote.

We allow this by introducing a new "promisor.checkFields"
configuration variable. It should contain a comma or space separated
list of fields that will be checked.

By limiting the protocol to specific well-defined fields, we ensure
both server and client have a shared understanding of field
semantics and usage.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>