Receiver\integration specific templates

[yuri-tceretian]

This PR proposes a more granular control over the templates for each receiver\integrations. The approach based on label matchers approach that is very common in alerting:

• Receiver definition is extended with supporting LabelSet.
• TempalateDefinition is extended with labels.Matchers
• Template factory is updated to select templates based on labels
• Receiver builder is updated to create template per integration. This allowed providing additional label to template factory "integrationType", which provides ability to pick template specific to a integration of a specific template kind.

Also, by adding specific labels to receivers we can merge many Mimir configurations into a single Alertmanager providing isolation for receivers and templates.