Add support for decimal logical type

[xgamer4]

Added in functionality for Decimal logical data types based of Python's decimal.Decimal class, added in fields and types.
Added in types and fields definitions for Decimal. Added in tests for fields and updated documentation. decimal.Decimal types can be declared in two ways:

class Example(AvroModel):
   implicit: decimal.Decimal = decimal.Decimal('3.14')  # scale = 2, precision = 3, inherit from default
   explicit: decimal.Decimal = types.Decimal(scale=2, precision=3, [default=decimal.Decimal('3.14'])

[xgamer4]

Little bit of reorganization of the added functions, added in serialization and schema tests, added in documentation, linted and tested.

Off the discussion in Issue 84, this was tweaked to error out on a decimal.Decimal field with no default. The docs also make it clear that defining scale and precision explicitly with types.Decimal is preferable to decimal.Decimal

[codecov-io]

Codecov Report

Merging #86 (e07efe0) into master (0a819c4) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #86      +/-   ##
==========================================
+ Coverage   99.50%   99.55%   +0.04%     
==========================================
  Files           7        7              
  Lines         608      672      +64     
  Branches       93      105      +12     
==========================================
+ Hits          605      669      +64     
  Misses          1        1              
  Partials        2        2              

Impacted Files	Coverage Δ
dataclasses_avroschema/fields.py	99.50% <100.00%> (+0.04%)	⬆️
dataclasses_avroschema/serialization.py	100.00% <100.00%> (ø)
dataclasses_avroschema/types.py	100.00% <100.00%> (ø)

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[marcosschroh]

Looking good @xgamer4 . Just minors comments