Optional support for file timestamps in NodeProperty

Context

Optional file timestamp support has been added to upstream REAPI: https://github.com/bazelbuild/remote-apis/pull/91 + https://github.com/bazelbuild/remote-apis/pull/102

This needs to be supported in buildbox-casd for incremental workspace builds in BuildStream.

Task Description

• CaptureTree implementation must support properties (at least MTime mtimes from CaptureTreeRequests) - this is essentially just a call to make_nesteddirectory with support for mtimes in the FileNodes
• support properties in StageTree implementation: ie. anticipate Files having properties while staging -> caller of LocalCasInstance::stage() -> FileStager::stage() -> FileStager::stageDirectory for FileStager::stageFile
• set file mtimes in staging

nb. this should be optional by virtue of the properties being optional in CaptureTree