Refactor and add BuildType interface

[ianlewis]

Depends on #69
Fixes #13
Fixes #2

Improve the API by adding the BuildType interface that builders can extend to implement their own build types. The intention is to improve the API and not rely so much on function parameters and allow the API to scale easier with minimal backwards incompatible changes.

The Github API client was added in order to get the workflow path via the API and solve #11 in a later PR.

The BuildType interface represents a SLSA build type and the GithubActionsBuild implements it with a basic Github Actions build type that can be extended by ecosystem builders.

A typical ecosystem builder might look like:

type GoBuild struct {
    GithubActionsBuild
}

func (b *GoBuild) URI() string {
	return "https://github.com/slsa-framework/slsa-github-generator-go@v1"
}

func (b *GoBuild) BuildConfig(context.Context) (interface{}, error) {
	return buildConfig{
		Version: buildConfigVersion,
		Steps: []step{
			// Single step.
			{
				Command: com,
				Env:     env,
			},
		},
	}
}

You can then create it and generate SLSA from it:

b := GoBuild{
	GithubActionsBuild: NewGithubActionsBuild(subjects, githubContext)
}
g := NewHostedActionsGenerator(&b)
p, _ := g.Generate(context.Background())

Github API and OIDC clients are a bit annoying to deal with in tests so the generator will support not using them by setting a NilClientProvider. In that case the generator might generate invalid SLSA but is good enough for automated or e2e tests.

if isTest() {
	g = g.WithClients(&NilClientProvider{})
}

[ianlewis]

I'll finish this up and send for review after #50 gets merged.

[ianlewis]

@laurentsimon Could you take a look at this? would like to merge at some point soon.

[laurentsimon]

LGTM, thanks!