AVRO-3540: Support dictionaries keyed by types other than string

[czf]

Make sure you have checked all steps below.

Jira

• [ ] My PR addresses the following Avro Jira issues and references them in the PR title. For example, "AVRO-1234: My Avro PR"
  • https://issues.apache.org/jira/browse/AVRO-3540
  • In case you are adding a dependency, check if the license complies with the ASF 3rd Party License Policy.

Tests

• [ ] My PR adds the following unit tests OR does not need testing for this extremely good reason:
• [ ] TestMapWithConverterSucceeds - tests new functionality
• [ ] TestMapWithoutConverterFails - covers maintain existing behavior as this is opt-in

Commits

• [ ] My commits all reference Jira issues in their subject lines. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Documentation

• [ ] In case of new functionality, my PR adds documentation that describes how to use it.
  • All the public functions and the classes in the PR contain Javadoc that explain what it does

[czf]

Whats the best way to get attention from a maintainer?