Raise errors if both externalService and service are set

[aaronandrino]

Description

Fixes #318

Type of change

• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

How has this been tested?

kind create cluster --name posthog
kubectx kind-posthog

# fails as expected
helm upgrade --install posthog-test \
	~/git/charts-clickhouse/charts/posthog \
	--set cloud=private \
	--set postgresql.enabled=true \
	--set externalPostgresql.postgresqlHost=test123.local

# works
helm upgrade --install posthog-test \
	~/git/charts-clickhouse/charts/posthog \
	--set cloud=private \
	--set postgresql.enabled=true \
	--set externalPostgresql.postgresqlHost=""

Checklist

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works

[macobo]

Minor request: please merge this file with charts/posthog/templates/error-on-removed-values.yaml to charts/posthog/templates/error-on-invalid-values.yaml, merge the tpl file with the other tpl file.

After that this looks great to me. It would be awesome to add some helm-unittest coverage here, but not required immediately.

[aaronandrino]

Done. I couldn't add an error test case because of this bug, but it should be possible to do so once the bug fix is released.

[macobo]

This is great! Thank you for the contribution 🙇