Support skipping CI for specific branches when pushing multiple refs

Proposal

The ci.skip option currently applies to the entire push. However, if a push contains multiple branch updates, only a subset of the branches or tags in question might want CI skipped. However, the push might not be able to be split due to the desire to use the --atomic flag to avoid refs not matching each other.

As a proposal, it would be nice if there were push options like branch.$name.ci.skip and tag.$name.ci.skip. Analogously for ci.variables. I glanced over the other push options available and these seem to be the only ones that would want this (IMO).