Add choice of "command interposer"

[DrJosh9000]

What

Change how podSpec/command and podSpec/args is interpreted. The choice of "interposer" is up to the user. If they allow the Kubernetes plugin, the choice can be made per-step, otherwise there is a default that can be set in values.yaml.

The default (if no choice is made) is changing.

buildkite mode is more convenient if you are used to pipeline.yaml syntax, and want to pass multiple commands as a list. vector mode is much more like how Kubernetes treats command and args, and is intended to execute a single command (that, per the Kubernetes docs, can be ["sh", "-c", "big script here"]. There's also legacy mode, equivalent to what it does before this PR, to ease transitioning pipelines over to new modes.

Why

Joining the container command and args with strings.Join(..., " ") was never exactly a good idea.

For example, before this PR, this doesn't do what it looks like it does:

podSpec:
  containers:
    - command:
        - echo First command
        - echo Second command

Because each command component was joined with spaces, this would run the command echo First command echo Second command, and echo First command echo Second command, not two things over two lines.

If we tried harder to interpret it the way Kubernetes does (as a raw arg vector executed directly, not a string interpreted by a shell), then we should be using the vector ["echo First command", "echo Second command"], which would fail because while echo could be an executable file (that takes one or more arguments), it's unlikely that echo First command is the name of an executable.

With no apologies to Terry Pratchet: If we're breaking Kubernetes' rules, we ought to break 'em good an' 'ard.

[wolfeidau]

@DrJosh9000 not a fan of the readme changes, was there a reason for this?

Happy to defer to others but without some sort of automated formatter this feels like it is just adding friction.

[DrJosh9000]

@wolfeidau it's not important here, so I'll back out the 100 column change.

[wolfeidau]

Looks good 👍🏻 🚀