Add support for amplify update function and EventSourceMappings

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**Is your feature request related to a problem? Please describe.**
I have a lambda function that acts as a DDB stream event handler. I'd like to be able to add multiple event sources to this function.

A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]

**Describe the solution you'd like**
I'd like to be able to run amplify update function and associate many @model DDB tables as event sources.

**Describe alternatives you've considered**
Manually changing the lambdas cloudformation to add the event sources.

**Additional context**
Add any other context or screenshots about the feature request here.