Add support for encoding and decoding Decimals

[jbruggem]

This PR proposes to add support for Decimal values in avro_ex.

As decimal values aren't accurately represented by native number in Elixir, it proposes to (optionally) decode these decimals losslessly as Decimal structs.

In order to ensure consistency with the official implementation, this PR proposes to test the decoding/encoding using a reference Avro encoded file generated with a Java implementation.

[jbruggem]

@davydog187 is there anything I can do to help with this ?

[davydog187]

@jbruggem looks good to me! Can you please fix dialyzer and the tests? Then we can get this released

[jbruggem]

@davydog187 thanks ! The compilation error is fixed.

[davydog187]

Thank you!