Impl MSC4461: Storing per-message profiles for users

[j0lol]

Description

Migrates PMP account data to the format described in MSC4461. Extensions to the format are defined in my informal MSCF002. (Though ambiguity behavior defined in this spec have not been implemented just yet.)

Part of #1279

Type of change

breaking change on an experimental feature, with migrations

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

AI disclosure:

• Partially AI assisted (clarify which code was AI assisted and briefly explain what it does).
• Fully AI generated (explain what all the generated code does in moderate detail).