Allow environment variables based on Fn::Base64

[ronkorving]

Is there an existing issue for this?

• I have searched existing issues, it hasn't been reported yet

Use case description

I so so wish I had spotted this during the conversation in #11409, because it would've probably saved a lot of time. My goal was to base64 encode a JSON structure that I pulled out of a YAML file (via {self:custom.} and use that as an environment variable.

Example:

environment:
  FOOBAR:
    "Fn::Base64":
      "Fn::ToJsonString": ${self:custom.foobar}

@jurriaanpro was kind enough to implement Fn::ToJsonString in #11461, but I completely missed that Fn::Base64 is not supported either in environment variables.

Proposed solution (optional)

#11671