Support enabled_tenants and disabled_tenants in alertmanager

[friedrichg]

Is your feature request related to a problem? Please describe.
enabled_tenants and disabled_tenants in rulers and compactors is a nice feature to run different ruler and compactors for different tenants.
It should be possible to do the same in alertmanager

Describe the solution you'd like
It should be possible to define -alertmanager.enabled_tenants and -alertmanager.disabled_tenants to enable specific tenants in the alertmanager cluster

Describe alternatives you've considered
None, I think this one is consistent with the rest of the options.

Additional context
None