check if chart values parse as yaml before installing application

[laverya]

What this PR does / why we need it:

Sometimes additional chart values will be invalid yaml. This will prevent the installation from completing 5+ minutes later with very hard to debug error messages. We should check for this early.

Which issue(s) this PR fixes:

Does this PR require a test?

Does this PR require a release note?

The 'install' command will check helm extension values for yaml validity before installation.

Does this PR require documentation?

[github-actions]

This PR has been released (on staging) and is available for download with a embedded-cluster-smoke-test-staging-app license ID.

Online Installer:

curl "https://staging.replicated.app/embedded/embedded-cluster-smoke-test-staging-app/ci/appver-dev-070059c" -H "Authorization: $EC_SMOKE_TEST_LICENSE_ID" -o embedded-cluster-smoke-test-staging-app-ci.tgz

Airgap Installer (may take a few minutes before the airgap bundle is built):

curl "https://staging.replicated.app/embedded/embedded-cluster-smoke-test-staging-app/ci-airgap/appver-dev-070059c?airgap=true" -H "Authorization: $EC_SMOKE_TEST_LICENSE_ID" -o embedded-cluster-smoke-test-staging-app-ci.tgz

Happy debugging!