Add an option for up-converting JSON data to Ion

[tgregg]

Currently, when the input file is JSON and one of the requested test formats is Ion, the data is converted to Ion via simple re-write using ion-java. This does basic numeric type conversions (numbers without decimal points are Ion integers, numbers with a decimal point but no exponent are Ion decimals, and all other numbers are Ion floats). Additionally, field names are automatically added to a local symbol table.

However, there are additional opportunities for up-conversion that would more closely match how you'd write JSON data using Ion's types. For example, because JSON does not have a native timestamp type, timestamps are often tunneled via strings. Timestamp-formatted strings could be detected and written as Ion timestamps. Recurring string values could be detected and added to the symbol table. Base-64 strings could be written as Ion blobs.

This should be added as an option named something like --json-to-ion-upconvert <bool> to allow users to control the behavior.