Support running built-in or external "programs"

[aplowman]

This PR adds some new attributes to Action that define running built-in or external executable "programs" (typically binary files). The immediate motivation for this is running compiled standalone Matlab applications (in particular compiled MTEX scripts).

The Action.program attribute (which corresponds to running a built-in program) is similar to the Action.script attribute, and shares some similar parametrisation. For example script_data_in/out -> program_data_in/out. For external programs, the Action.program_path attribute can be used. In future, I intend to reorganise the script-related code so built-in/external scripts are distinguished in this way as well.

This PR also add three new ElementResources attributes: platform, CPU_arch, and executable_extension, which cannot be set by the user but can be retrieved/referencde. The reason for adding these is so built-in programs can be organised by operating system type (i.e. the platform key, which takes values like "win" for Windows, "linux", and "macos"). executable_extension is .exe on Windows, and empty on other platforms.

This PR also includes an unrelated fix in how we store run IDs associated with jobscript blocks. We were storing run IDs as unsigned integers, but then expected signed integers elswhere, which caused problems (most obviously in the show command, for task schemas that have conditional actions)!