Opt-in updaters

Story: A user of Clair can opt-in to various matchers which are not enabled by default to pull in additional non-default updaters, e.g. those required CRDA remote matchers.

Acceptance criteria:

• there is a class of updaters that is not enabled by default but can be enabled via opt-in
• the CRDA updaters are part of above class
• absence of an opt-in updater leaves it disabled
• downstream release documents how to enables the CRDA updater as part of the default suggested Clair config
• Quay Operator by default enables the CRDA updater downstream

Product management note:

• this feature need to be code complete before OpenShift 4.9 feature freeze July 23rd 2021 to allow for enough testing until 4.9 final freeze by OpenShift QE