feat: support secret/configMap mounting via arbitrary volume sources

[C-nit]

Issue link

Closes #197

What changes have been made

• _helpers.tpl: while the claimName shorthand is kept, anything else passed through as a verbatim volume source
• values.yaml: reworded doc comment + four examples (PVC shorthand, verbose PVC, configMap, secret-with-items).
• README.md: match values comment
• tests/helloworld_test.yaml + .snap: the three new sources appended to the existing "Adding Volume Mounts" test; snapshot regenerated.

Verification steps

• extended unittest, passes
• compared compiled manifest to the result using the kustomize workaround mentioned in the linked issue.

Checks

• I've made sure the tests are passing.
• Testing Strategy
  • Unit tests
  • Manual tests
  • Testing is not required for this change

[C-nit]

https://github.com/helm-unittest/helm-unittest#install
I see the github actions need to either pin the installation of helm to version 3, or add --no-verify to the unittest plugin installation.

I wait for maintainer feedback before touching CI.

[dgrove-oss]

@C-nit -- please rebase to latest main to pick up a fix for CI.

You'll also have to fix the signature on your commit; hopefully you can do that without too much annoyance.

Thanks!

[dgrove-oss]

Thanks!