Extract structs related to creating alertmanager configuration (#163)

* Extract structs related to creating alertmanager configuration

Copied as is without any modification from grafana/grafana.

The purpose of this move is to make use of these structs directly in Mimir without having to import grafana directly.

For now, these are the ones I need but there might be more as I progress along.

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Signed-off-by: gotjosh <[email protected]>

Author: gotjosh