Add lookml validation to spoke-default and spoke-private

[scholtzan]

https://mozilla.slack.com/archives/C4D5ZA91B/p1710459680782959

In some cases it can happen that lookml validates correctly when using the Looker developer UI, but actually introduces lookml errors when deployed (for example, if looker-hub is not up-to-date during development). We should add a validation step to the spoke-default and spoke-private repositories to catch these errors before merging.

┆Issue is synchronized with this Jira Task