Alerting: Allow inserting before or after existing policy

[gillesdemey]

What is this feature?

This PR introduces the ability to insert a new policy before or after an existing notification policy.

We currently do not have the ability to re-order notification policies and prior attempts have shown to have some rough edges; re-ordering nodes in large trees was slow, not accessible and hard to figure out now that pagination has been added to the policy tree.

Instead this PR adds the ability to choose where to insert new policies.

A move operation would be a delete + insert at position, with the caveat that label matchers would have to be copied / memorised by the user somewhere.

We feel like this is an acceptable compromise considering that operations on the tree wrt to moving the order of policies is quite rare and often required because the policy they had just inserted got appended to the bottom of the child policies.

Which issue(s) does this PR fix?:

Fixes #60101

Special notes for your reviewer:

Needs a few tests before merging to make sure the insertBefore / insertAfter functions work as intended.

Please check that:

• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.

[soniaAguilarPeiron]

LGTM!🚀
No nits! tested locally and it looks great!
Cool feature!