feat(cdk-experimental): expose root loader instance in harness environment

[devversion]

Adds a new method that can be used by harness consumers to load a harness
through HarnessLoader from the document root. This is helpful for harnesses
which need to match elements outside of testbed fixtures (e.g. snack-bars, overlays etc.).

Needed for #16697 and #16709.

@mmalerba I just took a stab at implementing this as we discussed this on the PR (#16697 (comment)) and in the team meeting. I wanted to unblock my other harness PRs.

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[mmalerba]

CARETAKER NOTE: The new static method and its corresponding test also needs to be added to the CatalystHarnessEnvironment

[mmalerba]

#16709 was accidentally merged early and remove the protected on the constructor for TestbedHarnessEnvironment. Please add it back in this PR and switch the test to use the new method instead

[devversion]

@mmalerba Good catch. Updated!

[jelbourn]

@devversion could you rebase?

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