feat: add new api to unlock github-specific stage-level properties

[kaizencc]

Fixes #162.

Introduces a new API as follows:

import { App } from 'aws-cdk-lib';
import { ShellStep } from 'aws-cdk-lib/pipelines';
import { GitHubWorkflow } from 'cdk-pipelines-github';

const app = new App();

const pipeline = new GitHubWorkflow(app, 'Pipeline', {
  synth: new ShellStep('Build', {
    commands: [
      'yarn install',
      'yarn build',
    ],
  }),
  gitHubActionRoleArn: 'arn:aws:iam::<account-id>:role/GitHubActionRole',
});

pipeline.addStageWithGitHubOptions(new MyStage(this, 'Beta', {
  env: BETA_ENV,
  gitHubEnvironment: 'beta',
}));
pipeline.addStageWithGitHubOptions(new MyStage(this, 'Prod', {
  env: PROD_ENV,
  gitHubEnvironment: 'prod',
}));

app.synth();

Unlocks GitHub Environments at a stage-level, as well as a blueprint for any future stage-level github-specific properties we want to add.

[rix0rrr]

Looks good, minor comments. Feel free to merge afterwards.

[rix0rrr]

This feels potentially dangerous--multiple stacks can have the same name.

[kaizencc]

Changed the key to stack.artifactID which should be unique within the cloud assembly.

[julianiag]

@kaizencc, I just tried the new feature and found one thing.
If ppl use the normal AddStage method, the output deploy.yml will have environment: null. Wonder if we shall skip adding the environment key if this.stackEnvs[stack.stackArtifactId] is null.
Possibliy:

...(this.stackEnvs[stack.stackArtifactId] && {
          environment: this.stackEnvs[stack.stackArtifactId],
        }),

[kaizencc]

Hey @julianiag, caught me at a good time lol. Does that create an environment for you when you deploy? Or is it simply ignored? I agree it's not great. Maybe I can change it up tomorrow :).

[kaizencc]

Or, if you add that in as a PR I will approve it, but you'll have to run yarn build to update the snapshots.

[julianiag]

#186