promisor-remote: allow a server to advertise more fields

For now the "promisor-remote" protocol capability can only pass "name"
and "url" information from a server to a client in the form
"name=<remote_name>,url=<remote_url>".

To allow clients to make more informed decisions about which promisor
remotes they accept, let's make it possible to pass more information
by introducing a new "promisor.sendFields" configuration variable.

On the server side, information about a remote `foo` is stored in
configuration variables named `remote.foo.<variable-name>`. To make
it clearer and simpler, we use `field` and `field name` like this:

  * `field name` refers to the <variable-name> part of such a
    configuration variable, and

  * `field` refers to both the `field name` and the value of such a
    configuration variable.

The "promisor.sendFields" configuration variable should contain a
comma or space separated list of field names that will be looked up
in the configuration of the remote on the server to find the values
that will be passed to the client.

Only a set of predefined field names are allowed. The only field
names in this set are "partialCloneFilter" and "token". The
"partialCloneFilter" field name specifies the filter definition used
by the promisor remote, and the "token" field name can provide an
authentication credential for accessing it.

For example, if "promisor.sendFields" is set to "partialCloneFilter",
and the server has the "remote.foo.partialCloneFilter" config
variable set to a value, then that value will be passed in the
"partialCloneFilter" field in the form "partialCloneFilter=<value>"
after the "name" and "url" fields.

A following commit will allow the client to use the information to
decide if it accepts the remote or not. For now the client doesn't do
anything with the additional information it receives.

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

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