Support for extensions validation

[Damien Johnson]

Allow users to register custom validation logic in order to validate vendor extensions.

Custom validation logic is required since, by definition, vendor extensions are specific to the entity that defines them and may be validated in many different ways.

Implementation of this feature should minimally include:

• A way to register vendor extension validators
• A hook to call into registered vendor extension validators during the validation process

[James Navin]

Support for custom validation logic was added in v2.3.0. See README for details on how to configure it.

Please feel free to re-open this issue if it doesn’t meet your needs.