Beam Row Avro conversion: fixed and bytes decimals

An Avro decimal logical type annotates Avro bytes or fixed types.

Current Row to Avro conversion is limited to bytes type and in addition hardcodes precision to MAX_INT and scale to 0. 

I have attached a patch that adds support for decimal bytes and fixed types.

I could think of 2 possible ways to solve this:
- Change Row decimal type to a logical type
- Use Beam field options to pass additional metadata required to serialize decimals

I felt that overhauling Beam schema types might not be a good idea. Instead I went with option 2.

Passes ./gradlew check

Imported from Jira [BEAM-12877](https://issues.apache.org/jira/browse/BEAM-12877). Original Jira may contain additional context.
Reported by: kegelink.