Introduce a "promisor-remote" capability

Earlier this year, I sent 3 versions of a patch series with the goal of allowing a client C to clone from a server S while using the same promisor remote X that S already use. See:

https://lore.kernel.org/git/20240418184043.2900955-1-christian.couder@gmail.com/

Junio suggested to implement that feature using:

"a protocol extension that lets S tell C that S wants C to fetch missing objects from X (which means that if C knows about X in its ".git/config" then there is no need for end-user interaction at all), or a protocol extension that C tells S that C is willing to see objects available from X omitted when S does not have them (again, this could be done by looking at ".git/config" at C, but there may be security implications???)"

This patch series implements that protocol extension called "promisor-remote" (that name is open to change or simplification) which allows S and C to agree on C using X directly or not.

I have tried to implement it in a quite generic way that could allow S and C to share more information about promisor remotes and how to use them.

For now, C doesn't use the information it gets from S when cloning. That information is only used to decide if C is OK to use the promisor remotes advertised by S. But this could change in the future which could make it much simpler for clients than using the current way of passing information about X with the -c option of git clone many times on the command line.

Another improvement could be to not require GIT_NO_LAZY_FETCH=0 when S and C have agreed on using S.