Add reference tag to allow getting a varaible by reference

[vladoovtcharov]

Motivation

Adds the ability to reference a variable without making a copy, similar to that proposed in #1393

Instead of singleton: true being added to the object, instead anywhere that wants to be instantiated as a reference to that variable can use the syntax:
foo: bar: _target_: "myclass" x: 4.0 reference_variable: _reference_: "foo.bar"
Then reference variable and foo.bar will refer to the same object after instantiate.

By registering a custom resolver the syntax ends up looking even nicer:

foo: bar: _target_: "myclass" x: 4.0 reference_variable: ${ref:foo.bar}

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

Add test cases, especially for almost circular cases, e.g.
`
foo:
a: ${ref:bar.y}
b:
target: "ClassA"

bar:
x: ${ref:foo.b}
y:
target: "ClassB"
`
Also a bit worried about performance/memory usage with the proposed implementation but not sure about the best way to test that.

Related Issues and PRs

A different approach to PR#3103 but solves similar problem