First pass at alertmanager.jsonnet template

Ben Kochierequested to merge
bjk/alertmanager_jsonnet into master

Initial conversion of alertmanager Chef config to a jsonnet templte.

https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/10054

Signed-off-by: Ben Kochie bjk@gitlab.com

Edited by Ben Kochie

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