add a flag to ssm variables to resolve as string instead of object

[jeremygiberson]

A PR to serverless https://github.com/serverless/serverless/pull/5842/files changed how ssm variables were resolved for secrets manager. If the content of the secret was a valid JSON string, then it was parsed and returned as a JSON object. This causes issues when trying to use the secret in an environment variable.

This PR extends the extended syntax for ssm variables to add a directive decryptAsPlaintext to skip the json parsing so that the plain text value is returned.

This branch was created from tag 1.51.0. (as that is the version we were using at the time).
TODO:

• rebase from serverless/serverless master
• fill out template below
• make PR against serverless org

What did you implement

Closes #XXXXX

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Is this ready for review?: NO
Is it a breaking change?: NO

[jeremygiberson]

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