exclude_none and exclude_unset flags support added.

[dapper91]

Adds support for exclude_none and exclude_unset serialization flags.

• exclude_none - excludes model fields set to None:

>>> from typing import Literal, Optional
>>> from xml.etree.ElementTree import canonicalize
>>> 
>>> from pydantic_xml import BaseXmlModel, element
>>> 
>>> class Product(BaseXmlModel, tag='Product'):
...     title: Optional[str] = element(tag='Title', default=None)
...     status: Optional[Literal['running', 'development']] = element(tag='Status', default=None)
...     launched: Optional[int] = element(tag='Launched', default=None)
... 
>>> product = Product(title="Starlink", status=None)
>>> print(product.to_xml(exclude_none=True, pretty_print=True).decode())
<Product>
  <Title>Starlink</Title>
</Product>

• exclude_unset - excludes model fields that haven't been explicitly set:

>>> from typing import Literal, Optional
>>> from xml.etree.ElementTree import canonicalize
>>> 
>>> from pydantic_xml import BaseXmlModel, element
>>> 
>>> class Product(BaseXmlModel, tag='Product'):
...     title: Optional[str] = element(tag='Title', default=None)
...     status: Optional[Literal['running', 'development']] = element(tag='Status', default=None)
...     launched: Optional[int] = element(tag='Launched', default=None)
... 
>>> product = Product(title="Starlink", status=None)
>>> print(product.to_xml(exclude_unset=True, pretty_print=True).decode())
<Product>
  <Title>Starlink</Title>
  <Status></Status>
</Product>

Fixes the issues #203 and #194

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 96.15385% with 1 line in your changes missing coverage. Please review.

Project coverage is 91.83%. Comparing base (e928d4b) to head (bfd52b2).

Files	Patch %	Lines
pydantic_xml/serializers/factories/model.py	90.90%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #204      +/-   ##
==========================================
- Coverage   91.86%   91.83%   -0.03%     
==========================================
  Files          29       29              
  Lines        1610     1617       +7     
==========================================
+ Hits         1479     1485       +6     
- Misses        131      132       +1     

Flag	Coverage Δ
unittests	91.83% <96.15%> (-0.03%)	⬇️

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