Mark fields as readonly in docs

[jbolinger]

Marks the attribute as readonly in the generated docs if the OUTPUT_ONLY flag is set on the field.

I also added a field presenter test. Happy to add more tests as part of this PR, but I'm looking for advice on the structure. Should we have a folder under test/gapic/presenter/field, keep it with the method-level tests as I've done here since those presenters are fairly related, etc.? Alternatively, I can defer to #65 if you'd prefer to wait on adding these.

[resolves #103]

[quartzmo]

Should we have a folder under test/gapic/presenter/field, keep it with the method-level tests as I've done here since those presenters are fairly related, etc.?

I see that we have test/gapic/presenters/method_test.rb which contains MethodPresenterTest. Personally I think the file could be renamed to presenters/method_presenter_test.rb. Your test could be added in the same directory as presenters/field_presenter_test.rb.

One note, however, is that I might have expected the presenters path to be preserved:

templates/default/helpers/presenters/method_presenter.rb

and

test/templates/default/helpers/presenters/method_presenter_test.rb

But we've done a lot of moving things around already so don't take that as a very serious recommendation, just a nit.

[jbolinger]

Thanks @quartzmo - I moved the test alongside the method presenter test for consistency for now. We can always move it again later.

I also added a garbage test, but it doesn't cover all the bases yet. In particular, I couldn't figure out how to get the nested protos to pass but I think that can be addressed later.

[quartzmo]

Is anything else needed for this PR? I approved yesterday.

@blowmage Want to take a look?