Add URS `a` to config

[zahrajavar]

In the TRBFV public key contribution, the value a is shared across all parties. Move a from a circuit witness to a configuration parameter to guarantee consistency among participants.

[0xjei]

we are actively started working on this but decoupled from #1306

[0xjei]

this will involve a small refactoring of fhe & fhe-params crate - will complete at the end by tomorrow.