RFC: Post-W3F account and identity API surface

[filvecchiato]

Raised in: #222 · @filvecchiato

Description

Four post-W3F additions to the account and identity surface, tracked one RFC each.

Tasks

• RFC: Unified signWithAccount across product and legacy accounts
• RFC: getPrimaryDotNsName
• RFC: Account certifications
• RFC: Freebie claim account IDs under product keys

Note: the dotNsIdentifier discussion in #222's comments is tracked by #492 and #486, not here.