Support existing LBRY publisher on /youtube signup flow

[tzarebczan]

In order to support existing LBRY creators who migrate to YouTube Sync (non-public/internal api issue - https://github.com/lbryio/internal-apis/issues/471), we would need to capture additional data like their channel signing certificate (needs to be transferred/stored securely) and wallet address to start publishing their content automatically.

@alyssaoc we may want to create an epic (or turn this issue into an epic) if and when we are ready to spec it out.

[alyssaoc]

Issue moved to lbryio/ytsync #1 via ZenHub