Add builddecisionscript

[jcristau]

Port the build-decision code to run as a scriptworker task.

The following changes have been made:

• use taskgraph.util.keyed_by instead of a local copy
• input is taken from fields in the task payload instead of either command-line arguments or the environment
• we don't have a taskcluster proxy; instead we use temporary credentials obtained/maintained by scriptworker to create tasks / trigger hooks, and we use the github oauth token from the scriptworker config instead of from a taskcluster secret
• the CLI is modified to match other scriptworkers

Unlike other scriptworkers, we skip chain of trust verification.