AVRO-3217: Allow any identifier as annotation name

[opwvhk]

A field schema can be annotated with the property avro.java.string:

{
  "name": "fieldName",
  "type": {
    "type": "string",
    "avro.java.string": "String"
  }
}

The corresponding IDL is:

@avro.java.`string`("String") string fieldName;

The IDL parser fails on the `. This change fixes that.

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-3217
  • 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:
  Adjusted the simple.avdl and simple.avpr test

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

[RyanSkraba]

LGTM !

I've been away from the project for a few weeks -- thanks so much for your patience and for doing these IDL fixes (especially for doing them in these nice, digestible PRs). I should have a bit more time to get some reviews done!