[web] update encrypt method to return EncryptedData

Summary:
Context in ENG-7228.

Note: We use a shared olmAPI interface on both platforms, to split this logically into diffs sometimes I had to make changes on one platform and make some temporary changes on another platform to make sure CI passes but this approach is consistent and uses the same approach for both web and native.

Test Plan: Tested in D11451.

Reviewers: michal, marcin

Reviewed By: michal, marcin

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11445