Add `SpecialReportAltTheme` to `Format`

[DanielCliftonGuardian]

What does this change?

• Adds SpecialReportAltTheme to Format
• Adds a hashed special report alt tag for an upcoming special report

Part of the work for the new SpecialReportAlt theme: https://docs.google.com/document/d/1YnWmfEa1nQKEGgJbPgs9cp9VaMBOBILqGNRqqsZ0Jys/edit#

This PR is the 1st in this series:

1. content-api-scala-client: Adds SpecialReportAltTheme and SpecialReportAlt hashed tag to content-api-scala-client: https://github.com/guardian/content-api-scala-client/pull/371 (current)
2. facia-scala-client: Adds SpecialReportAlt to CardStyle: https://github.com/guardian/facia-scala-client/pull/280
3. frontend: Consumes the new SpecialReportAltTheme and adds SpecialReportAlt card styles for fronts rendered by frontend: https://github.com/guardian/frontend/pull/25602

How to test

• Tested locally using the SNAPSHOT version in facia-scala-client and frontend
• Tested locally with a unit test that when a specific tag is present the theme equals SpecialReportAltTheme

[paulmr]

If this is ready to go, are you ok to merge the PR and release the library @DanielCliftonGuardian ?