AVRO-2754: Add support for UUID logical types in C#

[mkellogg]

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-2754

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:
• Modified codegen test to add logical type tests. Nullable value types were also broken in the test so that fix sneaked in
• LogicalTypeTests.TestUuid - Verifies translation between serialized string and System.Guid

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

[Fokko]

Thanks for the contribution @mkellogg. Could you add the license?

[WARNING] Files with unapproved licenses:
  lang/csharp/src/apache/main/Util/Uuid.cs

[mkellogg]

Sorry about that. Added.

[blachniet]

Looks great! I had one change request. Thank you @mkellogg!