Bring PHP functionality in line with recent JS changes and add integration tests for third parties

[felixarntz]

Fixes #66.

This PR makes several enhancements:

• Add support for optionalParams for scripts, as was recently introduced in the JS codebase (see #58).
• Add third party implementation for Google Tag Manager, as it was recently added in the JS codebase (see #36).
• Add PHPUnit integration test coverage to ensure the output for all third parties works as expected (see #66).
• Fix various bugs that were exposed by the new integration tests.

It may be helpful for the review process to review the individual commits as part of this PR, rather than all changes at once.

Effectively, this PR does everything needed to bring it up to date with the JS codebase, and it includes test coverage to make sure in the future it doesn't get out of date without being noticed.

[felixarntz]

Pushed one more fix here in https://github.com/GoogleChromeLabs/third-party-capital/pull/70/commits/b38ca1214a4742d20037b83c7b3d699ba9eda2e3 because the schema changes from #71 broke PHPUnit tests again.