API: require means of querying list of email share recipients

[ASerbinski]

Is your feature request related to a problem? Please describe.
(https://github.com/nextcloud/server/blob/master/lib/private/Share20/Manager.php#L1670)
The problem is that the getAccessList function only returns a boolean for share by email recipients, I require a list of the actual email addresses to whom the Node is shared.

Looks like it boils down to this;
https://github.com/nextcloud/server/blob/master/apps/sharebymail/lib/ShareByMailProvider.php#L1208

Describe the solution you'd like
Instead of a boolean, return an array as it does for users and remote.