add BCP 47 language tag attribute for files

[jmartin-sul]

• optional because this won't be applicable for most files
• validated by the i18n gem if present
• initially useful for specifying the language of e.g. caption and transcript files

NOTE: Changes to openapi.yml require updating openapi.yml for sdr-api and dor-services-app and generating models - see README.

Why was this change made? 🤔

closes #637

How was this change tested? 🤨

unit tests

⚡ ⚠ If this change has cross service impact, run integration tests and/or test in [stage|qa] environment, in addition to specs. ⚡

[jmartin-sul]

merging since @mjgiarlo approved and i took his non-blocking suggestion.

not urgent, but still interested in feedback for possible follow on touchups re:

• docs/description_types.md after running exe/generator generate
• whether attribute? makes optional redundant
• whether to up the limit on (or disable) RSpec/ExampleLength

[mjgiarlo]

@jmartin-sul 💬

merging since @mjgiarlo approved and i took his non-blocking suggestion.

🙏🏻

• docs/description_types.md after running exe/generator generate

I think we ought to take whatever the generator generates (now and always---and if the generator isn't generating the right stuff, we should fix the generator logic).

• whether attribute? makes optional redundant

I believe attribute? makes the key optional (can be missing) and optional makes a value nilable?

• whether to up the limit on (or disable) RSpec/ExampleLength

I'm 💯 on-board with the change you made.

[jmartin-sul]

@jmartin-sul 💬

ok, took that output and added it in the first commit in https://github.com/sul-dlss/cocina-models/pull/641

🙏🏻

at any rate, the tests already ensure that the value can be omitted entirely, so that does seem to indicate that .optional is superfluous in some way in this case.

Thanks for testing this!