Add Custom Integration Key Type Using Expr Language

[mastercactapus]

What problem would you like to solve? Please describe: Currently, GoAlert does not provide support for arbitrary payload mapping to create alerts. This restricts the flexibility to use different payload formats for alert creation.

Describe the solution you'd like: A new "custom" integration key type that incorporates the Expr language to permit arbitrary payloads for alert creation would enhance the versatility of GoAlert. As part of this solution, summary, details, and dedup should be permitted to set an Expr expression. This expression would map a JSON or form-encoded payload into the alert fields.

A valid use case test could involve sending a Grafana payload and/or other integration-key type payloads with appropriate Expr values to create alerts.

Describe alternatives you've considered: Currently, no alternative solutions have been conceived due to the specific nature of this proposal.

Additional context: Following the successful testing and implementation of this feature, a future body of work could consider replacing the individual integration type code with a group of preconfigured expressions.