promisor-remote: allow a client to store fields

promisor-remote: allow a client to store 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.

Another previous commit, c213820c51 (promisor-remote: allow a client
to check fields, 2025-09-08), has made it possible for a client to
decide if it accepts a promisor remote advertised by a server based
on these additional fields.

Often though, it would be interesting for the client to just store in
its configuration files these additional fields passed by the server,
so that it can use them when needed.

For example if a token is necessary to access a promisor remote, that
token could be updated frequently only on the server side and then
passed to all the clients through the "promisor-remote" capability,
avoiding the need to update it on all the clients manually.

Storing the token on the client side makes sure that the token is
available when the client needs to access the promisor remotes for a
lazy fetch.

In the same way, if it appears that it's better to use a different
filter to access a promisor remote, it could be helpful if the client
could automatically use it.

To allow this, let's introduce a new "promisor.storeFields"
configuration variable.

Like "promisor.checkFields" and "promisor.sendFields", it should
contain a comma or space separated list of field names. Only the
"partialCloneFilter" and "token" field names are supported for now.

When a server advertises a promisor remote, for example "foo", along
with for example "token=XXXXX" to a client, and on the client side
"promisor.storeFields" contains "token", then the client will store
XXXXX for the "remote.foo.token" variable in its configuration file
and reload its configuration so it can immediately use this new
configuration variable.

A message is emitted on stderr to warn users when the config is
changed.

Note that even if "promisor.acceptFromServer" is set to "all", a
promisor remote has to be already configured on the client side for
some of its config to be changed. In any case no new remote is
configured and no new URL is stored.

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