[native] get signed prekeys
Summary: modify the existing generateAndGetPrekey function to return signed content and notif prekeys (prekey + signature)
Test Plan:
console logged generateAndGetPrekeys from account-selectors.js and got the following:
LOG {"contentPrekey": "{\"curve25519\":{\"AAAAAg\":\"9QT6LlKsgI+badfNPLMhlWon5GZjCBjFVTur/W/qRkY\"}}", "contentPrekeySignature": "T64w13LqXgw0M7F6d19qL5pQyfE+c86CdgNlckIPIQwiI+INDttUjxSNVaa9Vv0AMIqNC+2tq2NvzqTTPESCAg", "notifPrekey": "{\"curve25519\":{\"AAAAAg\":\"9cN8Gem9icUP869ydpukc9LkGXFj89ZZ1JT5IPma234\"}}", "notifPrekeySignature": "+CjQcbt4qKAMp2zQlAzg+zpJZ++OsLUjeMh9NuJVVoPNhPt7DEpPa1WEsYmH3hCEd8QqSxQM4lqH1mVvr3O3BA"}Reviewers: atul, marcin
Reviewed By: marcin
Subscribers: bartek, ashoat, tomek
Differential Revision: https://phab.comm.dev/D9084