Support environment variables file in S3

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**Tell us about your request**
What do you want us to build?

As a user I'd like to see a support for passing environment variables to App Runner service instances via env files in an S3 bucket (as ECS does). The original request is here: https://twitter.com/seiyakubo/status/1394848526341971971.

**Describe alternatives you've considered**
A clear and concise description of any alternative solutions or features you've considered.

It could be done by writing code.

**Additional context**
Anything else we should know?

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