feat(policy): Support marshalling of config

[iainlane]

Sometimes it's convenient to generate a Policy Bot config. For example, when handling required but optional workflows, you need the config file to contain a rule for each workflow which replicates its path filters.

That job is made much easier if the Policy Bot types can be reused. But without omitempty the generated YAML contains all fields and it's hard to read/review.

Add omitempty where it's needed, and IsZero in a few places too - which is how the behaviour of omitempty can be customised.

[palantirtech]

Thanks for your interest in palantir/policy-bot, @iainlane! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.

[bluekeyes]

Thanks, I think it's fine to support this for people who'd like to use it.

Note that we don't offer any guarantees about Go-level compatibility between different versions of Policy Bot. As long as the behavior of the running server is compatible, we may make breaking changes to any of the Go packages in minor/patch versions. Something to be aware of if you plan to consume this code in a tool to generate policies.

[iainlane]

Just remembered about this / came back to it and rebased 👍