Add sendable conformance to appropriate types

Add `Sendable` conformance to appropriate types would allow to enabled additional safety checks at compile time. Without it, developers have to guess whether a type is `Sendable` or not.

No files, tests, or specific types are named. Start by surveying the Swift Crypto public types and their existing protocol conformances, then identify which conformances are appropriate and validate that the project compiles with the additional Sendable safety checks.

Written by the indexing model from the issue text.