Feat: update item definition

[joshwarren]

Add update_definition endpoint, with corresponding update pipeline definition utility function.

[ladparth]

Thanks @joshwarren for the PR. I found one issue, Can you fix it before i merge it to dev?

The folderId should only be added to the payload when a folder is provided in create_item method. Currently it's always added even when folder is None, which will cause resolve_folder(None) to be called unnecessarily. Please update to:

if folder is not None:
    payload["folderId"] = resolve_folder(folder)

[joshwarren]

Sorry for the slow response and I've also realised I've been pushing from 2 different accounts. PR is now updated as per your feedback