Support OSFamily and ISA platform properties

Jürg Billeterrequested to merge
juerg/platform into master

This validates OSFamily and ISA platform properties specified by the client and enables use of linux32 to run x86-32 actions on x86-64 systems and aarch32 actions on aarch64 systems.

buildbox-run --capabilities is extended to advertise supported platform properties in the following format:

platform:OSFamily=linux
platform:ISA=x86-64
platform:ISA=x86-32
platform:unixUID
platform:unixGID

buildbox-worker and BuildStream can use this to detect supported platforms.

This depends on buildbox-common!251.

Edited by Jürg Billeter

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