CROM-6917 Optionally export MEM_SIZE and MEM_UNIT for all standard backends.

[kshakir]

No description provided.

[kshakir]

To effectively use "Retry with More Memory" for JVM jobs running on Papi one has to use MEM_SIZE and MEM_UNIT to run with the correct memory settings. But those variables were not available on any other backends. Therefore one ended up having to use other creative options for authoring multi-backend WDL such as using free.

This PR exposes those two environment variables to the command block on any Cromwell "standard" backend that supports the memory runtime attribute.

Using the environment variables also helps with call caching java jobs. One can use something along the lines of -Xmx${MEM_SIZE%.*}${MEM_UNIT%?} in a version 1.0+ WDL and the command block will stay the same even if the memory needs to be increased.

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Side note: If anyone comes across this PR and wonders why the default Local backend doesn't support MEM_SIZE and MEM_UNIT it's because the Local backend does not use memory (nor cpu at the moment). The memory runtime attribute would need to be added into the runtime attributes with something like:

runtime-attributes = """
  String? docker
  String? docker_user
  Int memory_mb = 2048
"""

And then inside submit-docker use --memory=${memory_mb}m.

Then the changes in this PR will generate MEM_SIZE and MEM_UNIT for the Local backend too.