WIP: custom platform properties

This is related to #1313

In short, there are remote-exection usecases where server/worker implementations may incorporate custom/additional platform properties, or choose not to implement all/any of the 'optional' standard properties, as defined by the platform message lexicon: https://github.com/bazelbuild/remote-apis/blob/master/build/bazel/remote/execution/v2/platform.md

• Add tests (probably excessive running the full set of tests against multiple platform configurations?)
• Add validation type filtering (no nesting etc)
• Add support for multi value properties (i.e, ISA which needs support adding already even without this MR)
• Cache Key considerations (when changing default properties which have previously constituted get_unique_key() for the sandbox config)

This branch is being exercised with this remote-apis-testing MR remote-apis-testing/remote-apis-testing!192