MSC3834: Opportunistic user key pinning (TOFU)

[BillCarsonFr]

Part of https://github.com/element-hq/element-meta/issues/764.

Establish a TOFU-style mechanism where users' signing keys are pinned when they're first encountered; internally, this will look similar to user verification, but will require no action on either user's part, and correspondingly will not be considered as high a level of trust.

As per MSC3834.