Add support for SSM Parameter Store parameters

[lazaroclapp]

The same underlying lambda extension allows access to AWS System Manager Parameter Store parameters (see docs here), in addition to secrets. However, as far as I can tell, the current version of this crate supports only secrets and not parameters.

This PR patches in support for parameters as well, including SecureString parameters (if the kms:Decrypt permission is available).

[lazaroclapp]

Let me know if this is a feature that should be part of aws-parameters-and-secrets-lambda and if so if any changes are needed from our internal patch above :) (Would have opened an issue first, but we already have this implementation for our own needs. We are just proposing to upstream it).

[Tom01098]

Hi @lazaroclapp, thank you for this PR! (And sorry it took me ages to get to this, GitHub didn't notify me 😢)

I'll review this shortly and get back to you :)

[Tom01098]

Thanks again for sharing this, it's a great feature to have in this package. This repo needs a little housekeeping which I'll do separately, then I'll release this as 1.0.0 :)

I'll also add a changelog and credit you for this.

Thanks!