[Ajv] Allow pass default and custom format validators

[igorbrasileiro]

Allow pass string formats to ajvResolver and use defaults from ajv-formats.

example schema:

{
  "type": "object",
  "properties": {
    "email": {
      "type": "string",
      "format": "email",
    },
  },
  "required": ["email"],
  "$schema": "http://json-schema.org/draft-07/schema#"
}