Extend mapping DSL to support Avro enum fields

[asnare]

At present our mapping DSL cannot map to values onto enum Avro fields. Mapping verification during startup detects a mismatch between the mapping and the Avro type.

This issue covers extending the DSL to allow string-based values to be mapped to Avro enumerations, so long as the string value corresponds to one of the values for the enumeration. (If it doesn't then the mapping would be absent.)

Tests will need to check:

• This works for a literal.
• This works for a string-based value provider.
• Unknown values are treated as absent.

[asnare]

Currently in most places within the mapping DSL any type conversions are explicit and of the form:

parse aField() as int

Given that this is a form of type conversion we should use the same sort of syntax. Something like:

parse theField() as enum

This makes it clear that the type conversion is expected is consistent with other type conversions that we support.