AVRO-2773: [C] Add decimal logical schema

[SahilKang]

AVRO-2773

What is the purpose of the change

These two commits add decimal logical schema support for C: the first commit adds the base layer and the second commit adds higher-level functions to manipulate decimal objects beyond their serialized representation.

Verifying this change

Existing tests were updated and new tests were added to exercise the new functionality.

Documentation

This introduces two new functions which are documented along with the existing functions.

For pre-existing functionality, the underlying bytes and fixed getter/setters can still be used with decimal objects.