docs: add ADR 0004 for work delegation interface

[dennislapchenko]

what

• adds ADR 0004, interface for delegating project commands out of Atlantis process
• docs only, no code

why

• Support executing terraform commands via k8s jobs #3791 is open since 2023 and every attempt on it went straight to one flavor. Last one was helm level proxy outside of codebase, never merged, covered only one VCS and one topology
• @pseudomorph asked on Support executing terraform commands via k8s jobs #3791 to define standard interface first and implement flavors after. This is that, written down so it can be argued before anybody writes code

What it decides:

• seam on project command (prjCmdRunnerFunc). Same granularity is already used by runProjectCmdsParallel, by project lock, by streaming jobID and by PlanStore keying. Per shell command and per whole command are rejected, with reasons
• Executor interface with LocalExecutor as default which changes no behaviour, RunID as fencing token, at-most-once contract for side effects
• own wire type instead of serializing command.ProjectContext, which carries logging.SimpleLogging and fields derived from VCS
• pluggable job output as hard prerequisite, pointing on feat: pluggable storage backends #6694 and Define data placement standards #6742 instead of inventing own selection mechanism
• no flavor is chosen, and four questions are left open on purpose so they are decided together with first flavor

tests

• docs only. markdownlint-cli2 with repo config passes

references

• Support executing terraform commands via k8s jobs #3791
• feat: pluggable storage backends #6694
• Define data placement standards #6742
• Highly available cluster with multiple nodes #1571