feat(conf): Add support for excludeMigrationPod to userDefinedVolumes and userDefinedVolumeMounts

[ashman1984]

What this PR does / why we need it:

At the moment Kong threats migrations PODs the same way as proxy PODs, and mounts all user defined PVs to both proxy and migration PODs.

While this might be useful in some situations, in most cases it is unnecessary to have user defined volumes mounted to migration PODs, and in some cases even not desired by the user.

This PR allows users to exclude user defined PODs from being mounted for migration.

This could be possible in the following ways:

1. Introduce additional property to _ userDefinedVolumeMounts_ and _ userDefinedVolumes_ to allows exclusion of user defined PODs from being mounted to migration jobs.

2. Obsolete current userDefinedVolumeMounts and _ userDefinedVolumes_ properties (located under root element "deployment") in favour of dedicated mounts for proxy and migration in a similar fashion as it is done for the ingress controller, where userDefinedVolumeMounts and _ userDefinedVolumes_ are defined under "ingressController" root property.

This might be done by adding userDefinedVolumeMounts and _ userDefinedVolumes_ under root section called "migrations" for migrations PODs, and for proxy under root section "proxy".

Special notes for your reviewer:

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

[pmalek]

Thanks for submitting the change. Can you please rebase on latest tip of main and add a ci values file so that we can test it in CI?

This will require running make test.golden.update to update the golden files.