Goal: Allow for more granular auto-pruning policies to target only certain image tags

Background: It is a common practice to version containerized software using image tags in a single repository and as such these repositories hold multiple version lines of the software (e.g. 2.x, 3.x, 4.x) but also images that resemble release candidates or nightly builds. Some customers also leverage tag naming pattern convention to denote images that are ready for production environments (eg. prod-..., stable-..., dev-...)
With today's auto-pruning policies it is not possible to target or exclude these image tags and the current policies are therefore perceived as too coarse.

Requirements:

• for organization-level and repo-level policies, allow to specify a regular expression to match a subset of tags for both existing policy types (tag amount, tag age)
• The expression should be optional and if left blank should be evaluated as "matching all images"
• the regular expression is user-supplied and needs to be protected against ReDOS attacks
• when updating to the Quay version implementing tag pattern support, existing policies should not be changed
• the registry-wide policy does not need that to support tag naming patterns at this point