Add approval and rejection support

[adamconnelly]

• Added the ability to approve and reject runs and tasks.
• The approval and rejection mutations accept an optional run ID (if not specified they target the stack blocker). Because of that I've created an extra optional run ID flag and renamed the required one.

Here's some example usages:

# Approving the current stack blocker
stack approve --id simple-stack

# Approving a specific run with a note
stack approve --id simple-stack --run "01GBDAFEF9HFBQW3ZPV3Z38K0P" --note "LGTM"

# Rejecting the current stack blocker with a note
stack reject --id simple-stack --note "Not on my watch"

# Rejecting a specific run with no note
stack reject --id simple-stack --run "01GBDAFEF9HFBQW3ZPV3Z38K0P"