Alerting: Fix provenance guard checks for Alertmanager configuration to not cause panic when compared nested objects

[yuri-tceretian]

What is this feature?
This PR fixes a panic that is caused when the provenance guard compares the settings of the current and submitted integrations that contain nested objects.

Why do we need this feature?
Currently, this makes it impossible to update non-provisioned contact points via regular API if the configuration contains provisioned PagerDuty integration. The request will cause panic

level=error msg="Request error" error="runtime error: comparing uncomparable type map[string]interface {}" stack="/usr/local/go/src/runtime/alg.go:249 (0x40cfd8)\n/drone/src/pkg/services/ngalert/api/api_alertmanager_guards.go:116 

Who is this feature for?
Users who provision PagerDuty integration because it is the only one that accepts nested objects in settings.

Which issue(s) does this PR fix?:

Fixes #

Special notes for your reviewer:

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.

[grafanabot]

Hello @yuri-tceretian!
Backport pull requests need to be either:

• Pull requests which address bugs,
• Urgent fixes which need product approval, in order to get merged,
• Docs changes.

Please, if the current pull request addresses a bug fix, label it with the type/bug label.
If it already has the product approval, please add the product-approved label. For docs changes, please add the type/docs label.
If the pull request modifies CI behaviour, please add the type/ci label.
If none of the above applies, please consider removing the backport label and target the next major/minor release.
Thanks!

[grafanabot]

Hello @yuri-tceretian!
Backport pull requests need to be either:

• Pull requests which address bugs,
• Urgent fixes which need product approval, in order to get merged,
• Docs changes.

Please, if the current pull request addresses a bug fix, label it with the type/bug label.
If it already has the product approval, please add the product-approved label. For docs changes, please add the type/docs label.
If the pull request modifies CI behaviour, please add the type/ci label.
If none of the above applies, please consider removing the backport label and target the next major/minor release.
Thanks!

[JacobsonMT]

LGTM 🚀