Set explicit input and output limits

[erwinvaneyk]

Currently Workflows does not have explicit input and output size limits (for Fission funtions). However, implicitly there are (for example: NATS has a 10 MB message limit), which causes unanticipated behavior---as the user is not aware of these limits.

To fix this, Workflows should have explicit message limits, and return an error immediately when the inputs or outputs exceed the limit.

cc @thenamly

[ghost]

• Default limit of NATS is 1 MB
• Google pub/sub: 10 MB
• Azure: 1 MB
• ...