This commit brings support for multiple secret keys in the TFHE
dialect. In particular, a parameterized `TFHE` circuit can now be
given as input, with any combination of (semantically valid) of
ks/bs/woppbs mixing different secret keys, and compiled down to a
valid executable function, with server keys properly looked up.
Secret keys are now stateful objects which can be:
-> none/unparameterized (syntax `sk?`): The keys are in state after
the lowering from the `FHE` dialect.
-> parameterized (syntax `sk<identifier, polysize, dimension>`): The
keys were parameterized, either by user or by the optimizer. The
`identifier` field can be used to disambiguate two keys with same
`polysize` and `dimension`.
-> normalized (syntax `sk[index]<polysize, dimension>`): The keys were
attached to their index in the list of keys in the runtime context.
The _normalization_ of key indices also acts on the ksk, bsk and pksk,
which are given indices in the same spirit now.
Finally, in order to allow parameterized `TFHE` circuit to be given
as input and compiled down to executable functions, we added a way to
pass the encodings that are used to encode/decode the circuit
inputs/outputs. In the case of a compilation from the `FHE` dialect,
those informations are automatically extracted from the higher level
informations available in this dialect.
