Builds and Artifacts API

[ejholmes]

This adds a Builds and Artifacts API so that external tools (like Empire) can trigger builds on demand and obtain the artifacts that result from those builds.

I've split this into two commits:

1. First commit contains a spec for the API using json-schema and auto generates docs and a Go client from it.
2. Second commit adds an implementation in server/api.

TODO

• Auth.
• Use github.com/pkg/httpx
• Add a default transport to the generated client to handle error resources.
• Move slack.branchResolver into the Conveyor struct, so sha can be optional in the builds API.
• Make branch an optional parameter when creating a new build?

[phobologic]

:+1:

[ejholmes]

Note to self. We should error out of Conveyor.Build if the GitHub repository hasn't been enabled, otherwise we could potentially build repos that haven't been explicitly enabled (e.g. a user makes a typo when using the /conveyor build slash command).