Add size limits to calls where arbitrary amount of data can be passed in

[saad-ali]

#83 (comment) adds a limit of 1 MB for credential data. We should consider something similar for other areas in the interface where an arbitrary amount of data can be passed in. It's always easier to start with a restriction and loosen it later then start with no limit and try to impose it later.

[jdef]

field size limits were added here: https://github.com/container-storage-interface/spec/blob/master/spec.md#field-size-limits

please re-open this if needed