[GoSDK + Prism] Support Process env execution.

[lostluck]

Process Env execution is another kind of Environment type that allows you to set a process to be started up as a worker.

Useful where docker isn't appropriate, for testing the bootloader directly.

The protocol for Process environments is nearly identical to the standard container contract. The process will be provided a --id and a --provision_endpoint flag, and otherwise be expected to handle the rest of the boot container logic.

Largely added for completeness prior to supporting the Any environment in Prism.

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[lostluck]

R: @damondouglas

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