mmv1: add FieldType() helper for documentation generation

[BBBmau]

implements a helper function first mentioned here:

• mmv1/docs: move write-only arguments into arguments section #15385 (review)
  after initial work on improving the documentation generation.

This moves the logic found within property_documentation.html.markdown.tmpl to its own helper method to handle the type of field when generating documentation (i.e output, write-only, required)

Validation on this is ensuring that we get no diffs when generating the docs

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[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR generated some diffs in downstreams - here they are.

google provider: Diff ( 3 files changed, 4 insertions(+), 4 deletions(-))
google-beta provider: Diff ( 3 files changed, 4 insertions(+), 4 deletions(-))

[melinath]

This should return a list of strings, which is joined in the template with the join helper, as described in #15385 (review).

Please also add unit tests.

[melinath]

removing service labels / review request since is a provider-wide change that just makes the docs a little easier to maintain.

[melinath]

The reason this should return a slice of strings is because we don't want to lock ourselves in to it being a string here; it limits the utility of the function. Additionally, it breaks the separation of concerns: the template is responsible for formatting the data we give it; handling that formatting in a helper method takes that responsibility away from the template (and makes it harder to understand what the final output of the template will be.)

There may be other helper methods that format & return strings to be directly inserted into the template, but that's generally a bad practice and we should remove them over time.

[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR generated some diffs in downstreams - here they are.

google provider: Diff ( 3 files changed, 4 insertions(+), 4 deletions(-))
google-beta provider: Diff ( 3 files changed, 4 insertions(+), 4 deletions(-))

[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR generated some diffs in downstreams - here they are.

google provider: Diff ( 3 files changed, 4 insertions(+), 4 deletions(-))
google-beta provider: Diff ( 3 files changed, 4 insertions(+), 4 deletions(-))

[melinath]

This is great, thanks for the thorough unit tests! The docs output does diverge slightly in that it adds "write-only" to things that are in the write-only section at the moment, but that's fine. It doesn't lose information, and your other PR is about to remove that section anyway!