feat: handle privilege escalation in boundary

[evgeniy-scherbina]

• Allow compilation on non-linux platforms. It's necessary because boundary will be part of coder binary, and it should be able to compile on different platforms.
• Handle privilege escalation in boundary binary instead of bash script.

It’s necessary to ensure that the coder boundary subcommand works correctly without requiring copy-pasting a privilege-escalation script into the Coder repository. Given that we support two types of jail, it also makes sense to let Boundary determine when privilege escalation is required and when it isn’t, which should make the experience simpler for users.