Introduce the following form input types to HumanInputNode:

• SELECT, which corresponds to a single select in form UI
• FILE, which corresponds to a form input accepting exactly one file
• FILE_LIST, which corresponds to a form input accepting zero or more files.

The SELECT form input type also introduces using runtime variables as the option source.

For example, a previous node could use HTTPRequestNode to query account list of a user, and the SELECT
input is used to allow the user to choose one account from the account list.