Rename and move `crypto.IBootstrapCrossSigningOpts` (#3352)
* Define `UIAuthCallback` type and use in `IBootstrapCrossSigningOpts`
* Move `IBootstrapCrossSigningOpts` to `crypto-api` and rename
* Replace uses of `IBootstrapCrossSigningOpts`
... with `BootstrapCrossSigningOpts`
* Update src/crypto-api.ts