Add $runtime argument for implementation specific information

[matthias-pichler]

What would you like to be added:

As discussed in #932 we will introduce a new runtime expression argument called $runtime that holds implementation specific information. It will include the following standard fields for now:

• $runtime.name ... the name of the runtime (e.g. Synapse, Sonanta, ...)
• $runtime.version ... the version of the runtime (e.g. commit sha, image hash, (semantic) version, ...)
• $runtime.metadata ... custom values that be set by implementors (e.g. tenantId, account, ...) alternatively we could use $runtime.implementation?

Future Additions

The following fields can be added in the future but haven't been decided yet:

Current Time

• $runtime.now.iso8601
• $runtime.now.epochMillis
• $runtime.now.epochNanos

Execution start times

An alternative proposed by @cdavernas was $workflow.startedAt ... which I prefer and propose in #951

• $runtime.executionStart.iso8601
• $runtime.executionStart.epochMillis
• $runtime.executionStart.epochNanos

Task start times

An alternative proposed by @cdavernas was $task.startedAt ... which I prefer and propose in #951

• $runtime.taskStart.iso8601
• $runtime.taskStart.epochMillis
• $runtime.taskStart.epochNanos

Operating System (suggested by @fjtirado )

• $runtime.os ... Personally not a fan because it might not always make sense/be known and might have unclear semantics inside run.container tasks

Why is this needed:

See #932

[fjtirado]

@JBBianchi Hey, I also suggested currentTime ;)
About operating system, I think is part of the shell and container discussion. We are assuming there is a shell available (that's the way to extend the functionality) and we are assuming containers can be run. So I definitely see there will be an operating system, running shells and containers.

[matthias-pichler]

I left out some of the things we haven't reached consensus on. We can discuss and potentially add them in the future 👍

Regarding run.shell: I thought this was going to be removed?

Regarding run.container: the os is either known from the image or can be determined by the code inside the container

Regarding currentTime: I think it is better suited for the $task and $workflow arguments.

[fjtirado]

@matthias-pichler-warrify Agree with the currenttime being on workflow and task objects.