Api 26837/shared schema updates for appeals

[caseywilliams]

Summary

Two updates to help bring appeals APIs in line with Lighthouse standards:

1. the non_blank_string schema has been renamed to nonBlankString. Initially we had renamed this in the opposite direction, because we were using underscores in the URLs and trying to be consistent with that. But now that we're complying with standards, I've renamed this back to its original name. I did this because although there's no specific nonBlankString guidance in the standards, they say that field names in general should be in camelCase (naming standard)
2. Adds an en-US Content-Language header (headers standard)

There are more changes to come for this ticket, but I figured this was enough for one PR

Related issue(s)

• API-26837

Testing done

• Updated existing specs and regenerated docs

What areas of the site does it impact?

• Segmented appeals APIs (these are not yet in production)

Acceptance criteria

• I fixed|updated|added unit tests and integration tests for each feature (if applicable).
• No error nor warning in the console.
• Events are being sent to the appropriate logging solution
• Documentation has been updated (link to documentation)
• No sensitive information (i.e. PII/credentials/internal URLs/etc.) is captured in logging, hardcoded, or specs
• Feature/bug has a monitor built into Datadog or Grafana (if applicable)
• If app impacted requires authentication, did you login to a local build and verify all authenticated routes work as expected
• I added a screenshot of the developed feature

[mathisto]

FWIW, I agree with your call on the field formatting and have been taking the same posture.