Add external dictionary operations

[gerrycampion]

Sample rule

Check:
  all:
    - operator: not_equal_to
      value: $valid_whodrug_codes
Operations:
 - id: $valid_whodrug_codes
   operator: whodrug_code_hierarchy
Rule Type: Record Data
Sensitivity: Record

Available operations:

• "valid_meddra_code_references" - Validates meddra codes.
• "valid_whodrug_references": Validates whodrug terms
• "whodrug_code_hierarchy": Validates whodrug codes are correct hierarchy
• "valid_meddra_term_references": Validates meddra terms
• "valid_meddra_code_term_pairs": Validates meddra terms match the supplied codes

AC:
Sample and available operations have been added to the documentation
Available operations have been added to the schema