feat: Add custom filter logic for stac-auth-proxy

[pantierra]

Closes: #370

Related: EOEPCA/resource-discovery#198 and IFRCGo/go-deploy#144

[pantierra]

This requires stac-auth-proxy to support extraVolumes and extraVolumeMounts: developmentseed/stac-auth-proxy#114

[pantierra]

#386 should fix the failing tests.

[pantierra]

I think a lot of the stuff should flow upstream: developmentseed/stac-auth-proxy#118
And then we can simplify this PR and make it nicer for the users.

[pantierra]

After implementing it upstream and adapting here to the new helm chart options, I ran into an important limitation: Files outside of a Helm application subchart, including those of the parent, cannot be accessed. This means we can't inject the custom_filters.py without the workaround (mounts and configMap) we were already using.

So i cleaned it up, added a few tests and this is ready for review.

[batpad]

So to make custom filters work I need to:

• Define the customFiltersFile
• Define extraVolumes as specified in the commented out bits
• Define extraVolumeMounts as shown in the commented out example

Is that roughly accurate? Can we make that a bit more explicit in the docs, that you need to specify customFiltersFile and ALSO specify the extraVolumes and extraVolumeMounts for it to actually work?

I can see that this is slightly awkward for good reason, but I think we could some-how be a little bit clearer in the comments here - does that make sense?

If I'm misunderstanding and you don't actually need extraVolumes and extraVolumeMounts, then ignore this comment :)

[pantierra]

Yes, you are right, we need all three things.

And you are also right that this should be much better documented. I went over the documentation and tried to make things much clearer.