Alerting: Add pagination and improved search for notification policies

[soniaAguilarPeiron]

What is this feature?

This PR adds several improvements to the policies view.

So far, if we have a big number of policies, the UI becomes very slow in this page. The interaction in this page becomes pretty irresponsive.

With this PR we introduce several things that fix this problem and also improve the user experience when searching.

• For each policy in the tree that exceed X number of policies, we add some pagination for its children with {numberofpoliciesHidden} additional policies button to expand the list of the branch that exceeds this X number
• When using the filters, the UI now shows a feedback of the results indicating if there are no matches or the number of policies that match the filters.
• When using filters, UI only show the nodes that belong to a matching path.

Why do we need this feature?

UI should support big number of policies.

Who is this feature for?

All users.

Which issue(s) does this PR fix?:

Fixes #73038

Special notes for your reviewer:

pagination.mp4

About the filter results:

filter-results.mp4

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.

[soniaAguilarPeiron]

/deploy-to-hg

[ephemeral-instances-bot]

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[soniaAguilarPeiron]

/deploy-to-hg

[gillesdemey]

Resolved @konrad147 's questions during 🍐-programming session

[konrad147]

LGTM!