[AVRO-4081][C++] Add big decimal support and update documentation

[glywk]

What is the purpose of the change

This pull request add support of c++ big-decimal logical type in Avro serializer/deserializer methods, related to AVRO-3779.

Verifying this change

This change added tests and can be verified as follows:

• Tests of schema serialization/deserialization with valid and invalid declarations.
• Specific validation on bad schema with precision and scale declaration.

Documentation

• Does this pull request introduce a new feature? (yes)
• If yes, how is the feature documented? (update of big-decimal logical type specification)