Make Pir/Pnns utilities Sendable

[fboemer]

Make EncryptionParametersConfig, PirDatabaseConfig, PnnsDatabaseConfig Sendable, so that we can omit nonisolated(unsafe) when creating custom benchmark configs.

E.g. we can now omit ``nonisolated(unsafe)` from the below:

nonisolated(unsafe) let encryptionConfig = EncryptionParametersConfig(
    polyDegree: 4096,
    plaintextModulusBits: [5],
    coefficientModulusBits: [27, 28, 28])

[karulont]

There are probably other things that should be Sendable.