[Python Schema] Python schema support custom Avro configurations for Enum type

[gaoran10]

Motivation

Currently, the Python client didn't support setting configurations required, default, required_default for Enum type in Record.

Modifications

Modify the _Enum class to CustomEnum class, the _Enum wasn't exposed to users, the new class CustomEnum will be exposed to users, they could set Avro definition configurations required, default, required_default.

How to use

class Color(Enum):
    red = 1
    green = 2
    blue = 3

class NestedObj(Record):
    a = Integer()
    color = CustomEnum(Color, required_default=True, default=Color.blue)

The schema definition will be like this

{
	'type': 'record', 
	'name': 'NestedObj', 
	'fields': [
		{'name': 'a', 'type': ['null', 'int']},
		{'name': 'color', 'default': 'blue', 'type': ['null', {'type': 'enum', 'name': 'Color', 'symbols': ['red', 'green', 'blue']}]}
	]
}

The old way of use will also be preserved.

This feature could work well with Java client.

Verifying this change

Make some changes in existing tests to verify this new feature.

Does this pull request potentially affect one of the following parts:

If yes was chosen, please highlight the changes

• Dependencies (does it add or upgrade a dependency): (no)
• The public API: (no)
• The schema: (no)
• The default values of configurations: (no)
• The wire protocol: (no)
• The rest endpoints: (no)
• The admin cli options: (no)
• Anything that affects deployment: (no)

Documentation

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• no-need-doc

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• doc

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[github-actions]

@gaoran10:Thanks for your contribution. For this PR, do we need to update docs?
(The PR template contains info about doc, which helps others know more about the changes. Can you provide doc-related info in this and future PR descriptions? Thanks)