Add status to pool invite

[CatalinVoineag]

Context

The invite form for a candidate to be invited by a provider to apply for a course will be DB backed, to make this work we need an enum to save the pool invite in a draft state before 'publishing' it after review step.

Guidance to review

Things to check

• If the code removes any existing feature flags, a data migration has also been added to delete the entry from the database
• This code does not rely on migrations in the same Pull Request
• If this code includes a migration adding or changing columns, it also backfills existing records for consistency
• If this code adds a column to the DB, decide whether it needs to be in analytics yml file or analytics blocklist, if included inform data insights team of the changes
• If this code adds a column that may include PII, the sanitise.sql script and 0025-protecting-personal-data-in-production-dump.md ADR have been updated
• API release notes have been updated if necessary
• If it adds a significant user-facing change, is it documented in the CHANGELOG?
• Attach the PR to the Trello card

[CatalinVoineag]

Candidate Pool - Build our proposed MVP ready for testing with Providers

[github-actions]

Database-level enum changes detected

Please include a data migration for these attributes and values:

0a1
> <enum> Pool::Invite.status: ["draft", "published"]