Add ZNE gate folding methods (random, from_front, from_back, etc.) as options

[ashsaki]

What is the expected feature or enhancement?
Allow users to choose from different ZNE gate folding methods.
For fractional noise factors, there are a several folding strategies available in the literature. Suppose, we have circuit with a total of $n$ (2-qubit) gates and need to fold $k$ gates to achieve a fractional noise amplification factor. Then, we can fold using following methods:

• random: randomly fold any $k$ gates.
• from first/front: fold first $k$ gates.
• from last/back: fold last $k$ gates.

Acceptance criteria
Maybe, a new field in ZneOptions to allow users select their choice of folding method.