refactor(entity): update types to interfaces

[alex-okrushko]

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PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ x] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Entities are using export type a lot. While they should work similar to interface, the latter is a preferred choice.
Moreover, tsickle tool has an issue type-annotating abstract classes that implement type at the moment: angular/tsickle#912

What is the new behavior?

The behavoir is unchanged, except it's more correct.

Does this PR introduce a breaking change?

[ ] Yes
[ x] No

Other information

[evmar]

I think even simpler is to write this:

type ComparerStr = (a: T, b: T) => string;

Here and below for the other function types.

(tsickle chokes on the old code, but I think the new code won't help there anyway, so you might as well write it in the simplest format, which tsickle also won't choke on.)

[alex-okrushko]

It takes new code, I verified it. But, yeah, for function types it looks even simpler with type