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

[omry]

Thank you for the PR. This is an interesting idea, and the ref syntax may be a useful direction for the broader shared-object / instantiate identity problem.

I am going to close this PR for now because this API has not had maintainer feedback or buy-in yet. Beyond the missing tests and news fragment, there are also fundamental design questions still open, including initialization order, how references interact with normal OmegaConf interpolation, and cycle detection / error semantics.

Please continue the design discussion on #1393 before opening a new implementation PR. This will not make it into the upcoming 1.4 release.