Add methods to get number of policies and templates in a PolicySet

[jv-garcia]

Description of changes

Add methods to get number of policies and templates in a PolicySet.

Regarding unit testing, I decided to just reuse this API in previous places where the size of existing iterators are used, it feels like more effort would feel silly here.

Issue #, if available

1179

Checklist for requesting a review

The change in this PR is (choose one, and delete the other options):

• [ ] A breaking change requiring a major version bump to cedar-policy (e.g., changes to the signature of an existing API).
• [ X] A backwards-compatible change requiring a minor version bump to cedar-policy (e.g., addition of a new API).
• [ ] A bug fix or other functionality change requiring a patch to cedar-policy.
• [ ] A change "invisible" to users (e.g., documentation, changes to "internal" crates like cedar-policy-core, cedar-validator, etc.)
• [ ] A change (breaking or otherwise) that only impacts unreleased or experimental code.

I confirm that this PR (choose one, and delete the other options):

• [ X] Updates the "Unreleased" section of the CHANGELOG with a description of my change (required for major/minor version bumps).
• [ ] Does not update the CHANGELOG because my change does not significantly impact released code.

I confirm that cedar-spec (choose one, and delete the other options):

• [ X] Does not require updates because my change does not impact the Cedar formal model or DRT infrastructure.
• [ ] Requires updates, and I have made / will make these updates myself. (Please include in your description a timeline or link to the relevant PR in cedar-spec, and how you have tested that your updates are correct.)
• [ ] Requires updates, but I do not plan to make them in the near future. (Make sure that your changes are hidden behind a feature flag to mark them as experimental.)
• [ ] I'm not sure how my change impacts cedar-spec. (Post your PR anyways, and we'll discuss in the comments.)

[jv-garcia]

One of the checks failed, is this a known issue? I'd probably need a bit of help to fix it if needed.

[cdisselkoen]

It's a problem unrelated to the change in this PR. Given that, happy to merge this now -- looks like we have 2 approvals and the CI failures are unrelated.