Don't use enums for synth strategies

[Ectras]

Enums represent a closed set of options and prevents users from using their own synthesizers. Instead, the synthesizers should be passed as generic arguments. For instance,

pub struct PauliExponentialSynthesizer {
    pauli_strategy: PauliPolynomialSynthStrategy,
    clifford_strategy: CliffordTableauSynthStrategy,
}

should probably be

pub struct PauliExponentialSynthesizer<PS, CS> {
    pauli_synthesizer: PS,
    clifford_synthesizer: CS,
}