feat: proxy avatars for Accounts_AvatarExternalProviderUrl

[anicoa]

This PR adds the ability to proxy external avatars. Solves Issue #32812

Internal ticket: https://rocketchat.atlassian.net/browse/CONN-298

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[anicoa]

@tassoevan sorry if I get it wrong. But now - as the pr is merged now - where can i activate the proxying of the external avatar url through the rocketchat server itself? Can't find the feature I added the pr for in the admin section anymore ;)

[tassoevan]

@anicoa We discussed internally if a setting made more sense than just always proxying all the avatars requests. As we didn't find any strong reasons for maintaining the old behavior, the setting was ditched while the new behavior was kept.

[anicoa]

@tassoevan thanks for clarification and work. Found this right now in the code but could not reproduce the behaviour on a fresh updated 6.12 installation. Will try/investigate and report.

[anicoa]

@tassoevan checked the functionality with one 6.12.1 and one 6.12 installation.

1. Set Accounts_AvatarExternalProviderUrl to https://www.google.com/{username}
2. Login
3. Developer-Tools show request to https://www.google.com/foobar

Anything else I have to consider?