Add the contact form submission resource base class

Problem/Motivation

Form submission endpoints exist on all three sites, differ only in which form and which fields, and carry the module's most security-sensitive code: validation, flood control and mail. They also carry an undeclared dependency — the core does not store contact messages, so a site that expects to find them in the database finds ContentEntityNullStorage.

Proposed resolution

• MyrestContactFormResourceBase declaring the form id, the payload-to-field map, the required fields and the mail behaviour, with everything else in the base.
• Validation and sanitisation through the mechanisms the module already has, and the same flood control.
• The storage question answered explicitly: whether submissions are stored, and if so where, said in the documentation rather than assumed.
• The request body examples kept working — getOpenapiBodyExample() stays part of the contract.

Remaining tasks

Everything.