promisor-remote: allow storing and using fields

Previous work 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. It's also 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 and then use these additional fields passed by the server. 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.

In the same way, if it appears that a different filter is better when using a promisor remote, it could be helpful if the client could automatically use it.

This is related to #623