Add `encoding` selector option (base64 or binary)

[andsel]

Release notes

Add encoding option to select the encoding of Avro payload between base64 (default) and binary

What does this PR do?

Adds encoding set option to choose the encoding of Avro payload. By default it's set to base64.

Why is it important/What is the impact to the user?

Let the user to decide whether to use binary or base64 encoding.

Checklist

• [x] My code follows the style guidelines of this project
• [ ] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [ ] I have made corresponding change to the default configuration files (and/or docker env variables)
• [x] I have added tests that prove my fix is effective or that my feature works

Author's Checklist

• [x] run this gem in Logstash

How to test this PR locally

• clone this branch and configure Gemfile
• execute the pipeline:

  
  input {
  generator {
  message => '{"id": "my account", "amount": 5.0}'
  count => 1 
  codec => json
  }
  }

output { stdout { codec => avro { schema_uri => "/tmp/avro_schema_payment.asvc" base64_encoding => false } } }

- Avro schema to save in  `/tmp/avro_schema_payment.asvc`

{"namespace": "io.confluent.examples.clients.basicavro", "type": "record", "name": "Payment", "fields": [ {"name": "id", "type": "string"}, {"name": "amount", "type": "double"} ] }

## Related issues

<!-- Recommended
Link related issues below. Insert the issue link or reference after the word "Closes" if merging this should automatically close it.

- Closes #123
- Relates #123
- Requires #123
- Superseeds #123
-->
- Closes #30

## Use cases
As user I want to receive the binary Avro payload without base64 encoding.

[andsel]

I would ask if @karenzone could give a quick check to the documentation change

[andsel]

Published with https://rubygems.org/gems/logstash-codec-avro/versions/3.4.0-java