Expose symmetric encryptions

[samtay]

On the outside, all this does is expose extra encrypt_symmetric_* methods wherever there were encrypt_* methods, on both the Runtime and the LogProofBuilder. Internally I've changed a few things

• Gotten rid of the runtime::BfvEncryptionComponents. This lived at the runtime level and contained the higher level plaintext and ciphertext types that are supposed to be agnostic to seal, however it also contained types specific to seal. And it kind of forced a wrapping and unwrapping of seal data, when logproof itself is already seal specific. So instead, just define and deal with types AsymmetricComponents and SymmetricComponents from the seal_fhe layer. The builder interacts with these as the runtime iterates over the inner plaintexts.
• Added a marker to the seal_fhe::Encryptor to track whether or not it supports sym/asym encryptions.

See Sunscreen-tech/SEAL#8 for seal changes.

[ryanorendorff]

Looks good! The cleanup of the components and splitting into the AsymmetricComponents and SymmetricComponents is a nice abstraction.