AVRO-3760: [python] Fix resolution of future enum with default symbol

[aiven-anton]

What is the purpose of the change

Parsing an enum should resolve to the default symbol when the symbol is not known to the reader. This raised a SchemaResolutionError. The patch adds handling of default symbols, see AVRO-3760.

Verifying this change

The change adds tests that verifies that the behavior is now in line with the spec, but retains the current behavior when there is no default symbol.

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable