Add schema support for strings as sample indices in `output_type_id`

[elray1]

Per discussion related to #48, in a first pass we will only support integer sample indices for output_type_id, but we would eventually like to support strings as well. To do this, we will need to add two properties to the output_type_id field in a hub's tasks.json config file:

• “type”: the data type of the sample index. "string" or "integer"
• “max_length”: if "type" is "string", the maximum number of characters in a sample index

"max_length" should be required if "type" is "string", but not otherwise.

[nickreich]

relating this to issue #70

[annakrystalli]

This is superceeded by #70 correct? My understanding is that there is no output_type_id property for samples anymore and all this is specified in output_type_id_params?