NEW: Add extension hook for field-specific validation

[lozcalver]

Depends on https://github.com/silverstripe/silverstripe-framework/pull/10569

[lozcalver]

Would I need to update composer.json to "silverstripe/framework": "^4.12" as part of this? I seem to remember we used to do that automatically for minor releases, but I think that stopped?

[michalkleiner]

Would I need to update composer.json to "silverstripe/framework": "^4.12" as part of this? I seem to remember we used to do that automatically for minor releases, but I think that stopped?

That would be my expectation to require a version that introduced what's used here, unless we update this PR to include a $this->hasMethod('extendValidationResult') check.

[lozcalver]

Removed the composer.json update as both PRs are now targeting 5.0.0

[GuySartorelli]

Rebased on top of 2.0 to get rid of the unrelated CI failures.