implement data api item functions

[jreiberkyle]

implement the following item functions in the SDK

• get_item
• get_item_type
• list_item_types

#501 depends

[mkshah605]

@jreiberkyle Some clarifying questions:

• Is get_item referring to item_get in our CLI interface docs?
• Does get_item_type mean the ITEM_TYPE argument within item_get?
• Also where does the list_item_types come in?

[jreiberkyle]

@mkshah605 this is for the python api, that is, the client functions.