feat(yaml): Support extVar to yaml command

What

feat(yaml): Support extVar to yaml command

Support passing the --ext-str option to the yaml command just like in the test command.

Also support passing --ext-code

Why

We'd like to load a different recordingRuleRegistry when generating mimir rules compared to Prometheus / Thanos rules as in gitlab-com/runbooks!6874 (merged)

Test

❯ cat test.jsonnet
{
  'test.yml': '{%s: %s}' % [std.extVar('key'), std.extVar('value')],
}
❯ go run main.go yaml --ext-str key=hello --ext-code value=1+1 test.jsonnet
cat test.yml
❯ cat test.yml
hello: 2