Switch instantiate target filtering to an allowlist model for 1.4

[omry]

Tracking issue for the Hydra 1.4 line.

The instantiate blocklist is defense-in-depth and cannot be a complete security
boundary. For 1.4, move hydra.utils.instantiate() toward a stricter
allowlist-based model for selecting _target_ values.

Proposed direction:

• Users declare trusted module patterns that Hydra is allowed to instantiate,
  for example:

  hydra:
    instantiate:
      allowlist:
        - my_app.*
        - sam2.*
        - hydra_plugins.*

• The allowlist must come from trusted application code/configuration, not from
  untrusted config being instantiated.

• Block or reject targets outside the allowlist with an actionable error.

• Point the error to migration documentation.

• Document how applications that consume untrusted or semi-trusted config should
  configure allowed targets.

This is expected to be a breaking change and should include migration docs.