Implement native FMN attack

[lucascionis]

Implement the FMN attack with the native backend. The implementation should allow custom choice of loss, optimizer, and other customizable components.

Original attack paper:

• FMN https://arxiv.org/abs/2102.12827

References:

• the base attack class https://github.com/pralab/secml-torch/blob/main/src/secmlt/adv/evasion/base_evasion_attack.py
• the PGD implementation https://github.com/pralab/secml-torch/blob/main/src/secmlt/adv/evasion/modular_attack.py