feat: Add the ability to monitor a registry with a tag filter

[jghiloni]

Somewhat related to the conversation(?) in #151, this PR provides the pipeline author a way to monitor a registry image for new tags, and to use that for the get criterion. This has been tested against public and private repos.

[jghiloni]

Let's call this WIP for now -- I realized this doesn't help for anything other than check scripts, I want to be able to handle pinning, as well as adding the tag to the in metadata

[vito]

@jghiloni I think right now this doesn't really address #151 - which is fine, it doesn't have to, and what's here is still useful since it allows you to specify what is effectively still a fixed tag to track - except it works by determining the tag via a regexp and using semver ordering to pick the latest one that matches, rather than specifying a static tag value.

In contrast, #151 is almost an entirely different versioning model, where it would return each semver tag as its own version in semver-order. In that model, if you wanted to pin to a specific version you could say version: {tag: 1.2.3} in image_resource, on a get step, or in the resource definition (alongside source:). You might still want something like tag_filter to filter the versions that it returns though.

[vito]

@jghiloni Just FYI, #151 has been on my weekend to-do list for a while so I'm taking a swing at it now! Just thought I'd ping in case you had also started looking into it beyond what's done here so far. Happy to hold off if you are.

[jghiloni]

Nope, go ahead! This has had to take a backseat.

[vito]

@jghiloni #214 is just about ready if you'd like to take a look! Curious to know whether the semver pattern matches your use case.

[jghiloni]

It's exactly our use case, would love to give it a shot when you get a chance.