Ability to create secret values from parts

Some open source containers require the DSN to be a single string:

```
POSTGRES_URI=postgres://foo:SECRET@host
```

But this is not possible to achieve from a single RDS cluster secret, which is a key-value JSON.

It'd be great if the `Secrets.[].ValueFrom` could support this functionality and allow crafting a secret environment variable value out of parts.

Thanks.

No files, tests, or entry points are identified in the issue. Start by determining which AWS container service owns Secrets[].ValueFrom and how RDS cluster secrets are exposed; done means a secret environment variable can be composed from JSON secret parts into a single DSN.

Written by the indexing model from the issue text.