Add "dashboard" namespace prefix

[rmccue]

For Dashboard-specific endpoints, we should add a new namespace prefix of dashboard/

Proposal

[rmccue]

Discussion in Slack for the API meeting.

• wp/dashboard/v1 fits with the existing reserved namespace of wp, but doesn't fit with precedent
• wp/ is a strong signal that the endpoints are part of core, but we already have oembed/
• dashboard/ would be for internal, dashboard-only endpoints not for use by general API clients
• Core is basically split into two parts, core stuff, and "Dashboard App" specific pieces
• Dashboard endpoints will probably be used outside of dashboard context no matter what we do
• wp/ can be too strong a signal, since we're not modelling core data types and it might not be RESTful anyway
• "admin" vs "dashboard": "dashboard" is an intentionally broader term, covers things like the Customiser which aren't in the admin, but are part of the "Dashboard App"

[rmccue]

WP40748 tracks the addition of an endpoint for the Community Events data in the dashboard, which would use this namespace.