Add Entity::new_with_tags() and Entity::tag()

[cdisselkoen]

Description of changes

Adds public methods Entity::new_with_tags() and Entity::tag() as discussed in #1374.

Discussion points:

• I made new_with_tags() more generic than the existing Entity::new(); we can take any IntoIterator for the attrs and parents parameters, there's no reason we need to force callers to have HashMap and HashSet specifically. Theoretically I believe we could adjust the Entity::new() signature in the same way without breaking semver if we wanted to keep these aligned.
• There's also no real reason we need owned String for attribute and tag names; the implementation immediately converts to SmolStr anyway, so we could take any ToSmolStr for instance. I haven't made that change in the current version of this PR because I'm not sure how much we want to keep SmolStr and related types/functions as an implementation detail.

Issue #, if available

Fixes #1374

Checklist for requesting a review

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

• A backwards-compatible change requiring a minor version bump to cedar-policy (e.g., addition of a new API).

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

• Updates the "Unreleased" section of the CHANGELOG with a description of my change (required for major/minor version bumps).

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

• Does not require updates because my change does not impact the Cedar formal model or DRT infrastructure.

I confirm that docs.cedarpolicy.com (choose one, and delete the other options):

• Does not require updates because my change does not impact the Cedar language specification.

[john-h-kastner-aws]

I made new_with_tags() more generic than the existing Entity::new(); we can take any IntoIterator for the attrs and parents parameters, there's no reason we need to force callers to have HashMap and HashSet specifically. Theoretically I believe we could adjust the Entity::new() signature in the same way without breaking semver if we wanted to keep these aligned.

I do prefer the new signature. Changing the old signature would probably break type inference in some edges cases if we want to take a strict definition of semver.

There's also no real reason we need owned String for attribute and tag names; the implementation immediately converts to SmolStr anyway, so we could take any ToSmolStr for instance. I haven't made that change in the current version of this PR because I'm not sure how much we want to keep SmolStr and related types/functions as an implementation detail.

No strong opinion of whether SmolStr should be part of our API, but it doesn't seem like a problem to me. A quick search finds one non-experimental function that already exposes it:

cedar/cedar-policy/src/api/id.rs

Line 64 in d625809

pub fn escaped(&self) -> SmolStr {

[cdisselkoen]

I think I'll leave this API using String to keep things a little more consistent between new() and new_with_tags()