[6.x] REST API: resolve entries by slug

[jackmcdade]

Summary

• GET /api/collections/{collection}/entries/{id} also accepts a slug.
• ID still wins if an id and another entry's slug collide.

Stacked on #15320.

Test plan

• /entries/about works when the id is not about
• If an entry id is about and another slug is about, the id wins
• Unpublished slugs still 404
• Existing id lookups unchanged