mixture protocol restructured and serial concatenation implemented

[Zshan0]

As mentioned in #4876, the mixture protocol has been restructured.

• The serial concatenation is handled by converting the decomposed operations into their superoperator form with the help of cirq.Moment().expand()
• To convert from kraus to mixture, eigenvalue decomposition is done to obtain the corresponding probability.

Merging this PR would clear the blockage for #4822.

[Zshan0]

mixture protocol has been restructured with serial concatenation as a part of it, serial concatenation uses superoperator as intermediate after decomposing the given operator. If any of the decomposed operators do not have a mixture then the given operator also is considered to not have a mixture.