[ChengShi-1]

What this PR does / why we need it:

Adds use cases for the new sign-up API, so the modern UI (and other clients) can let new users create a builtin account and optionally connect their ORCID iD while signing up.

• signUp calls POST /users/signup without authentication and creates a builtin user.
• getOrcidSignUpAuthorizationUrl calls GET /users/signup/orcid/authorize and returns the ORCID URL to send the user to.
• connectOrcidForSignUp calls POST /users/signup/orcid with the code and state ORCID returns. It gives back the ORCID iD, the details to pre-fill the form with, and the orcidToken to include in signUp.
• AuthenticatedUser has an optional authenticatedOrcid.
• ReadError and WriteError now expose the HTTP statusCode and, when the API returns them, per-field fieldErrors. That lets a form show validation errors next to the right fields.
• ApiRepository.doPost takes an optional authRequired argument (default true), so a request can be sent without the configured credentials.

This replaces 3f94643b, which was pushed to develop by mistake and is being reverted in #501.

Which issue(s) this PR closes:

• Closes #

Related Dataverse PRs:

• Depends on Add a sign-up API for the modern UI, with optional ORCID iD dataverse#12858

Special notes for your reviewer:

• Please don't merge or publish until Add a sign-up API for the modern UI, with optional ORCID iD dataverse#12858 is merged. The endpoints only exist there, behind the dataverse.feature.api-builtin-user-signup feature flag.
• The integration tests call the new endpoints, so they fail against the current unstable Dataverse image with a 404. They need an image built from Add a sign-up API for the modern UI, with optional ORCID iD dataverse#12858.
• test/environment/docker-compose.yml turns on the feature flag and turns off the sign-up rate limit for the integration tests.

Suggestions on how to test this:

1. Unit tests: npm run test:unit.
2. Integration tests (need a Dataverse image built from Add a sign-up API for the modern UI, with optional ORCID iD dataverse#12858): set DATAVERSE_IMAGE_TAG in test/environment/.env to that image, then run npm run test:integration.
3. The ORCID use cases need an ORCID sandbox client, so the integration tests only cover the cases that don't reach ORCID, such as ORCID not being configured.

Is there a release notes or changelog update needed for this change?:

Yes, added under "Unreleased" in CHANGELOG.md.

Additional documentation:

The three new use cases are documented in docs/useCases.md.