Extend mitq.shadows beyond Cirq circuits

[andreamari]

Issue Description

The code of mitiq.shadows is now implemented but it just works with Cirq circuits. We need to extend it to all Mitiq-compatible frontends.

Proposed Solution

Use the tools in mitiq.interface to make mitiq.shadows multiplatform.

Hint: check how this problem has been solved for other techniques (mitiq.xxx) and see if something similar can be applied in mitiq.shadows .

[natestemen]

Link to papers related to shadows:

• https://arxiv.org/abs/2011.09636
• https://arxiv.org/abs/2002.08953

cc @bdg221 @Aaron-Robertson since both of y'all were interested in getting more familiar with the shadows theory.