Allow Velero to support multiple credentials per storage provider

[nrb]

Describe the problem/challenge you have
Currently, Velero can only have 1 credential for a given storage platform, which is limiting if backups should be sent to different locations. For example, if someone is using GCP and wants to send backups to 2 different object storage buckets, this is not currently supported.

Describe the solution you'd like
Velero should offer some way to allow all storage provider platforms the ability to utilize multiple credential files. This should work for object storage only. Handling of volume snapshots will not be changed and they will continue to use the existing credentials provided during install.

Anything else you would like to add:
AWS can get around this by specifying profiles in the credential file, but not all file formats offer this support.

Also, restic support should be taken into account as it is currently part of Velero's core, not a plugin.

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[zubron]

Fixed in #3442, #3489, and #3559.

[zubron]

Note: updated issue description to remove reference to volume snapshots as this will only affect object storage.