[Pre-1.0] Change Send to not accept variadic list of msgs

[abraithwaite]

Send currently looks like this:

kawa/types.go

Line 113 in cac41ab

Send(context.Context, func(), ...Message[T]) error

After extensive use internally, I feel strongly that we should change that to remove the variadic nature of msgs.

	Send(context.Context, func(), Message[T]) error

We simply haven't made use of the multi-message send enough, and the ability to omit messages entirely calling send is awkward and leads to weird questions around implementation like "do I have to handle the empty messages case?"

The new API will support evolution through adding functional variadic options without breaking builds later on, if they become necessary.

Most importantly, it would make implementing destinations far far simpler.