Use the new backend APIs for adding-3pid-to-homeserver and binding-3pid-on-identity-server when they exist.

[lampholder]

I believe this replaces #2654.

Defined in matrix-org/matrix-spec-proposals#2290

• Detect whether HS supports MSC2290 (look for unstable flag m.separate_add_and_bind or API version r0.6.0)
• Always show account 3PID section in Settings (even without IS) if the HS supports MSC2290
• Add SDK support for POST /_matrix/client/r0/account/3pid/bind, POST /_matrix/client/r0/account/3pid/unbind, and POST /_matrix/client/r0/account/3pid/add
• Add SDK support for POST /_matrix/identity/v2/validate/<medium>/requestToken
• Implement new 3PID add flow for MSC2290 servers
• Implement new 3PID bind flow for MSC2290 servers
• Use /unstable prefix for APIs

[jryans]

See also Riot Web version: element-hq/element-web#10839