Aws secrets manager simple strings and error checking

[nickhammond]

String-based secrets

SecretString can technically be a regular string value and not a JSON object, this PR adds support to fallback to the simple string value when JSON parsing fails.

https://docs.aws.amazon.com/secretsmanager/latest/apireference/API_SecretValueEntry.html

You still fetch the value out the same way as a JSON-based secret but from there you can process however you need.

SECRETS=$(kamal secrets fetch --adapter aws_secrets_manager --from PG_PASSWORD)

PG_PASSWORD=$(kamal secrets extract PG_PASSWORD $SECRETS)

Errors

I also noticed that errors are not returned with a return code from the CLI but within the Errors array. Before this PR it just returns an empty secret value for an unknown secret, this PR checks the Errors array and prints out a message(from AWS) for each error.

Current output:

$ kamal secrets fetch --adapter aws_secrets_manager unknown-secret
\{\}

Updated output:

$ kamal secrets fetch --adapter aws_secrets_manager unknown-secret
ERROR (RuntimeError): unknown-secret: Secrets Manager can't find the specified secret.

https://docs.aws.amazon.com/cli/latest/reference/secretsmanager/batch-get-secret-value.html (Output -> Errors)