Bug 1976779 - rsclientcerts: make each backend responsible for rate-limiting calls to find_objects r=jschanck
Before this patch, `rsclientcerts::manager` would rate-limit calls to
`find_objects` to once every 3 seconds because the underlying operation can be
time-consuming (in particular, on macOS and Windows, if there are many
certificates/keys available). On Android, keys aren't available until the user
selects one, which means that if a call to `find_objects` happened before the
selection prompt was shown (which is what happens) and the user chose one in
less than 3 seconds, the backend wouldn't search again, thus making it seem like
no keys were available, which would cause Firefox to not send a client
certificate. This patch makes each backend implementation responsible for this
rate-limiting, because only they know if it's appropriate to do so (in particular,
on Android, `find_objects` doesn't have the same performance concern as
on macOS and Windows because rather than searching for certificates and
keys, it asks `ClientAuthCertificateManager` for the cached list of certificates
and keys that have already been approved for use by the user).
Differential Revision: https://phabricator.services.mozilla.com/D257065
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