support specifying custom certificates when accessing remotes

[skshetry]

something like certs in the config for the remotes instead of using AWS_CA_BUNDLE/REQUESTS_CA_BUNDLE, etc.

Related: #3450, treeverse/dvc.org#1079

[skshetry]

Closing as not planned. Looks like there is not much of an interest, and since we are using fsspec now, different filesystems will have different requirements, so we have to implement this on a per-filesystem basis.