Introduce defaults for SSM parameters

[marcoreni]

Is your feature request related to a problem? Please describe.
I'd like to be able to add fallbacks to SSM Params, so that if the param does not exist we can use a default.

Describe the solution you'd like
when declaring the parameters, there should be the possibility to set a default for each value.

Describe alternatives you've considered
Declaring the default at runtime

Additional context

[willfarrell]

Can you share the use case that the parameter would not exist? Typically this middleware, and similar, are for loading in shared configurations or secret that need to exist before the handler is triggered.