feat(controller) support user-defined mounts and bump version

[rainest]

What this PR does / why we need it:

Allow user-defined controller mounts, largely to enable usage of the functionality added in Kong/kubernetes-ingress-controller#2258

Also bumps Kong version to 2.8 because it's a chore and I didn't want to bother with a separate PR.

Which issue this PR fixes

(optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged)

• fixes Add support for controller volume mounts #543

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• PR is based off the current tip of the main branch.
• Changes are documented under the "Unreleased" header in CHANGELOG.md
• New or modified sections of values.yaml are documented in the README.md
• Commits follow the Kong commit message guidelines

[shaneutt]

Overall looks good, just some small comments.

Also I'm sure you've tested this manually but it occurs to me that we should be able to add an integration test for this fairly simply, that seems like something we should do: the most obvious test would seem to be to add certs to a mount in the test and verify they are mounted properly and in use.

[rainest]

Stuffing emptyDir mounts into the test values is sufficient for what I want to test here: we trust that Kubernetes does indeed mount the volume given well-formed volume and mount configuration, so for our part we just want to ensure that the template inserts the user blocks into the appropriate locations. If we don't, Kubernetes rejects the Deployment as invalid (the tests don't show this because it's a failure state, but it very much did happen earlier in development when I screwed up indentation and variable references).

[shaneutt]

I think we need to talk more soon about testing, but that doesn't need to hold this up.