Consider deprecating assignment of strings to non-string nodes like IntegerNode / FloatNode

[odelalleau]

Is your feature request related to a problem? Please describe.

Currently OmegaConf accepts the following string assignment:

@dataclass
class Foo:
  x: int = 10

cfg: Foo = OmegaConf.structured(Foo)
cfg.x = "20"  # OmegaConf happily converts from string to int, though mypy would complain

Describe the solution you'd like

Deprecate this functionality to increase type safety. More precisely, all assignments that mypy would complain about should raise a deprecation warning (and, later, an error)

Additional context

This issue was written following the discussion here: https://github.com/omry/omegaconf/pull/445/files#r541981251

[omry]

Note that this behavior is well documented in 2.0 and the docs will have to reflect this change.
This extends to boolean as well.
Note that merge should still support it.