Entity type refactor (simplified)

[markylaing]

@tomponline @hamistao this is a much more concise entity type refactor that I think works much better. (Thanks Tom for the idea of keeping the types as they are, but storing them in a map the returns a struct/interface with relevant fields/methods).

With this change, I think it should be easy to add the URL prefix classification by:

1. Adding a PrefixMatch() []string method to typeImpl that returns a slice of URL prefixes to match the entity type against.
2. Adding a function MatchURLPrefix which iterates over the entityType map, and iterates over the PrefixMatch() array for each typeImpl, then returns the entity type if strings.HasPrefix(u.Path, prefix).

Closes #13262
Closes #12928

[tomponline]

@markylaing can we mark this one as "closes #13262"? and "closes #12928"?

[hamistao]

@tomponline @markylaing Entities will need to be grouped to classify endpoints for the API metrics, grouping them on the metrics package makes sense but I think it would fall out of date eventually because one would have to update the metrics package as weel when adding an entity type that did not fall within the previous categories. Do you think it would be appropriate to extend this implementation in the futute with a field on typeInfo (could maybe be called EntityDomain or ResourceClass, not sure yet) specifying the group on which that entity type belongs?

The groups themselves would also be created within the entity package entity

Edit: This comment can just be disregarded as we agreed upon just using the approach outlined in the PR description.

[tomponline]

This looks good, a couple of nits then ready to go!

[hamistao]

As far as the metrics go, this look good. Thanks for this, will help a lot!

[tomponline]

Thanks!