feat: implement secret-to-bgv lowering

[asraa]

Lower secret.generic's containing single tensor_ext / add / mul / extract to bgv dialect.

cc @j2kun

Components:

• [x] #528
• [x] #565
• [ ] #825
• [x] #826
• [ ] #827

And potentially also:

• #266
• #658

[AlexanderViand-Intel]

I'm resurrecting this issue as a parent issue/tracking issue for ongoing improvements to --secret-to-bgv