[Swift] Adds a serialize helper function to native table

[mustiikhalil]

The following adds the function serialize to the object-api classes:

changes:
1- adds func serialize() -> ByteBuffer { return serialize(type: flatbuffers_object.self) } into the generated code

2- creates the helper methods:

public func serialize<T: ObjectAPI>(type: T.Type) -> ByteBuffer where T.T == Self
/// Allows devs to pass in their own builders, in case they have them in a singletons, instead of reallocating the space each time
public func serialize<T: ObjectAPI>(builder: inout FlatBufferBuilder, type: T.Type) -> ByteBuffer where T.T == Self

where the second serialize function gives the developer more control over the builder that's being passed to it
@jackflips

Closes #6058