Support logicalTypes

[smyrman]

Support setting logicalTypes in scema, e.g.:

var floatAvroSchema = avro.MustParseSchema(`{
		"type": "record",
		"name": "FloatRecord",
		"fields": [
			{ "name": "time",  "type": "long", "logicalType": "timestamp-micros"},
			{ "name": "value",  "type": "double"}
		]
}`)

Cutrrently, when parsed, the "logicalType" key is dropped, as there is no field for it in SchemaField.

This is useful e.g. when importing data to Google BigQuery with --use_avro_logical_types:
https://cloud.google.com/bigquery/docs/loading-data-cloud-storage-avro#logical_types