Alerting: Add OAuth2 Support for Webhook Receiver (#106302)

* Add to available channels

* Export

* Fix bug in deeply nested secrets

BE: Slice re-use bug when traversing deeply.

FE: Only at most one level of nesting was being taken into account
when determining secureFields keys. This change adds a new field on
NotificationChannelOption: secureFieldKey. This is populated on API GET via
transform. This change gives us the option to hardcode secureFieldKey in the
backend and no longer calculate the key via settings topology.

* Update grafana/alerting to 3e20fda3b872

* Prettier

* Linting

* Fix IntegrationConfig test to catch secure field mismatch

go.sum

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+github.com/grafana/alerting v0.0.0-20250610043455-3e20fda3b872 h1:bhUBPxMHtQB+AJaPcASElgwC7WFjWQvEiD3FoMEAv0E=
+github.com/grafana/alerting v0.0.0-20250610043455-3e20fda3b872/go.mod h1:VANfU4LMnOR4Gv4hkCN9fGyQ/8Lf2DKOvzG70vI2IS4=
 github.com/grafana/authlib v0.0.0-20250515162837-2f4a8263eabb h1:oTl2j6/4miQUYmXANp2pBuYCWA5f8NVYFfCWpczpFso=
 github.com/grafana/authlib v0.0.0-20250515162837-2f4a8263eabb/go.mod h1:PBtQaXwkFu4BAt2aXsR7w8p8NVpdjV5aJYhqRDei9Us=
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