feat(abc): src abc variables command

[bahrmichael]

This PR introduces a src abc variables command for setting and deleting variables on workflow instances using the update variables api from Sourcegraph.

Syntax:
src abc variables delete <workflow-instance-id> <variable-name>
src abc variables set <workflow-instance-id> <variable-name>=<variable-value>

With multiple arguments and --var:
src abc variables delete <workflow-instance-id> --var <variable-name-1> --var <variable-name-2>
src abc variables set <workflow-instance-id> --var <variable-name-1>=<variable-value-1> --var <variable-name-2>=<variable-value-2>

Examples:
src abc variables delete QWdlbnRpY1dvcmtmbG93SW5zdGFuY2U6MQ== approval
src abc variables set QWdlbnRpY1dvcmtmbG93SW5zdGFuY2U6MQ== prompt="Make some follow up changes"

The workflow instance id is an argument after the delete/set command, so that we can follow the existing subcommand patterns without having to introduce support for prefix args.

[bahrmichael]

List of notable changes since review:

• Added help message on UsageError (I just got "exit code 2" if I forgot the instance id; we can isolate this to just the new commands, but I thought it's helpful to have this in general)
• Implemented proper support for -get-cli (tested locally, If I understand correctly this flag is evaluated by the api client, and then we're supposed to just do a noop if ok=false)
• Coalesced args and flag var args and simplified the methods
• Added DisableSliceFlagSeparator so that lists won't be split into multiple args