Add option to overwrite transfer URLs with a client's base URL. (#306)

Needed because FromData and InitializeUpload methods sometimes ignore
custom endpoints set by the client.

Test environment was a bit broken, but Python 2.7 tests passed. Python 3
tests had a few unrelated failures probably related to the fact that my
Python3.5 version was having SSL issues. Ended up running the tests with
Python3.9. Failures:

```
ERROR: Testing pickling and unpickling of FieldList instances.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/local/google/home/hartunian/Desktop/apitools/apitools/base/protorpclite/messages_test.py", line 511, in testPickle
    unpickled = pickle.loads(pickle.dumps(field_list))
  File "/usr/local/google/home/hartunian/Desktop/apitools/apitools/base/protorpclite/messages.py", line 1147, in extend
    self.__field.validate(sequence)
AttributeError: 'FieldList' object has no attribute '_FieldList__field'

======================================================================
ERROR: Testing pickling and unpickling of Message instances.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/local/google/home/hartunian/Desktop/apitools/apitools/base/protorpclite/messages_test.py", line 1876, in testPickle
    unpickled = pickle.loads(pickle.dumps(message))
  File "/usr/local/google/home/hartunian/Desktop/apitools/apitools/base/protorpclite/messages.py", line 1147, in extend
    self.__field.validate(sequence)
AttributeError: 'FieldList' object has no attribute '_FieldList__field'

======================================================================
FAIL: Test decoding improperly encoded base64 bytes value.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/local/google/home/hartunian/Desktop/apitools/apitools/base/protorpclite/protojson.py", line 324, in decode_field
    return base64.b64decode(value)
  File "/usr/lib/python3.9/base64.py", line 87, in b64decode
    return binascii.a2b_base64(s)
binascii.Error: Invalid base64-encoded string: number of data characters (17) cannot be 1 more than a multiple of 4

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/local/google/home/hartunian/Desktop/apitools/apitools/base/protorpclite/test_util.py", line 70, in assertRaisesWithRegexpMatch
    function(*params, **kwargs)
  File "/usr/local/google/home/hartunian/Desktop/apitools/apitools/base/protorpclite/protojson.py", line 214, in decode_message
    message = self.__decode_dictionary(message_type, dictionary)
apitools.base.protorpclite.messages.DecodeError: Base64 decoding error: Invalid base64-encoded string: number of data characters (17) cannot be 1 more than a multiple of 4

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/local/google/home/hartunian/Desktop/apitools/apitools/base/protorpclite/protojson_test.py", line 441, in testDecodeBadBase64BytesField
    self.assertRaisesWithRegexpMatch(
  File "/usr/local/google/home/hartunian/Desktop/apitools/apitools/base/protorpclite/test_util.py", line 75, in assertRaisesWithRegexpMatch
    self.assertTrue(match, 'Expected match "%s", found "%s"' % (regexp,
AssertionError: False is not true : Expected match "Base64 decoding error: Incorrect padding", found "Base64 decoding error: Invalid base64-encoded string: number of data characters (17) cannot be 1 more than a multiple of 4"

======================================================================
FAIL: testConvertIdThatNeedsEscaping (apitools.base.py.batch_test.BatchTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/local/google/home/hartunian/Desktop/apitools/apitools/base/py/batch_test.py", line 360, in testConvertIdThatNeedsEscaping
    self._DoTestConvertIdToHeader('~tilde1', '<%s+%%7Etilde1>')
  File "/usr/local/google/home/hartunian/Desktop/apitools/apitools/base/py/batch_test.py", line 352, in _DoTestConvertIdToHeader
    self.assertEqual(
AssertionError: '<30037b18-267d-4e07-9ac8-bc3c8da9a959+%7Etilde1>' != '<30037b18-267d-4e07-9ac8-bc3c8da9a959+~tilde1>'
- <30037b18-267d-4e07-9ac8-bc3c8da9a959+%7Etilde1>
?                                       ^^^
+ <30037b18-267d-4e07-9ac8-bc3c8da9a959+~tilde1>

```