chore: Add git sha and url overrides

[avangelillo]

What does this PR do?

Add the ability to override the git sha and repo url to make snapshot testing easier.

Motivation

Requested by a customer in #359

Testing Guidelines

• Unit tests

Additional Notes

This PR allows you to override the git commit sha without having to modify your stack code, other than making the datadog lambda public on your stack. From a code snippet from the issue, adding the new line will make the snapshot consistent

test('Snapshot', () => {
  const app = new App();
  const stack = new SomeStack(app, 'test', {
    env: {
      account: '0123456789012',
      region: 'us-east-1',
    },
  });

  // New line
  stack.datadogLambda.overrideGitMetadata("fake-sha");

  const template = Template.fromStack(stack);
  expect(template.toJSON()).toMatchSnapshot();
});

Types of Changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
• This PR collects user input/sensitive content into Datadog

[lym953]

This is same as the previous test case. Is this expected?

[avangelillo]

Good catch, fixed