Missing Sendable Conformance in Core Types (Field & Reference) Breaks Concurrent Usage

[vamsii777]

Describe the bug
Missing Sendable conformance in core MongoKitten types (Field and Reference) causing compiler errors when used in concurrent contexts. The generic types Field and Reference were marked as Sendable but didn't properly handle the Sendable requirements of their generic parameters.

To Reproduce

1. Use MongoKitten in a project with strict concurrency checking enabled
2. Create a model using @Field property wrapper with any Codable type
3. Use Reference with any ReadableModel
4. Compile the project
5. Observe compiler errors about Sendable conformance

struct Project: Model, ResponseEncodable {
    @Field var _id: ObjectId
    @Field var organizationId: Reference<Organization>
    @Field var name: String
}

Expected behavior

• Field should be Sendable only when its wrapped type C is Sendable
• Reference should be Sendable when M.Identifier is Sendable
• No compiler errors when used with Sendable-conforming types
• Clear compiler errors when used with non-Sendable types

Environment:

• OS: macOS 15.1.1 (24B91)
• Swift Version: 6.0
• MongoKitten Version: 7.9.6