feat: The json module is now avaiable to stream maps (#2531)

* Add json function to CustomStreamMap

This allows using `json.loads()` and `json.dumps()` to convert fields
between text and json representations.

* docs: Add json function to Stream Maps

* Update docs/stream_maps.md

* Update docs/stream_maps.md

* Update docs/stream_maps.md

* Update docs/stream_maps.md

---------

Co-authored-by: Edgar Ramírez Mondragón <16805946+edgarrmondragon@users.noreply.github.com>

Author: grigi

docs/stream_maps.md

@@ -234,6 +234,9 @@ can be referenced directly by mapping expressions.
     `hashlib.md5(<input>.encode("utf-8")).hexdigest()`.
 - `datetime` - This is the datetime module object from the Python standard library. You can access
     datetime.datetime, datetime.timedelta, etc.
+- [`json`](inv:python:py:module:#json) - This is the json module object from the Python standard
+    library. Primarily used for calling [`json.dumps()`](inv:python:py:function:#json.dumps)
+    and [`json.loads()`](inv:python:py:function:#json.loads).
 
 #### Built-in Variable Names
 

singer_sdk/mapper.py

@@ -12,6 +12,7 @@
 import fnmatch
 import hashlib
 import importlib.util
+import json
 import logging
 import typing as t
 
@@ -306,6 +307,7 @@ def functions(self) -> dict[str, t.Callable]:
         funcs["md5"] = md5
         funcs["datetime"] = datetime
         funcs["bool"] = bool
+        funcs["json"] = json
         return funcs
 
     def _eval(