and the return value

Description
-----------
This worklog introduces :

- The mechanism to allow UDF authors to specify the character set of the UDF
  arguments during UDf initialization. Server hands over converted UDF
  arguments to the UDF() during execution.

- UDF authors can also specify the character set of the return value while
  initializing the UDF. During UDF execution, UDF author can convert the
  return value into the specified charset name of return value.

- If invalid charset names are specified then UDF initialization fails.

- Specifying the charset name of UDF arguments and return value is done by
  utilizing the extension pointer in the  UDF_INIT and UDF_ARGS structures
  without breaking the ABI compatibility.

- A server component service "mysql_udf_metadata" is added that sets/gets the
  values to/from the extension pointers. The service has following methods.

  (1) argument_get()
  (2) argument_set()
  (3) result_get()
  (4) result_set()

- Improved some of the existing UDFs in the audit_log, firewall, data_masking
  and keyring_udf plugins.

Testing
-------
- Added a daemon plugin and a component to test the feature. Refer the
  following test files.
  + test_udf_extension_services_plugin.test
  + test_udf_extension_services_component.test

Review
-------
RB: 22353