Add secret_prefix var so that one can grant MWAA access to secrets under a specific prefix

[Almenon]

See https://github.com/aws-ia/terraform-aws-mwaa/pull/23. There is a need for MWAA to have access to certain secrets so it can connect to various API's. Granting MWAA access to all secrets would be a security concern, as it should only have access to the secrets it needs, so we should grant it access to secrets under a specified prefix.

[vara-bonthu]

See the comment on PR. This can be achieved by using additional iam policies .