server: allow splitting and reassembling a StartHandshake

[lucab]

This adds two methods to the existing StartHandshake in order to allow splitting and reconstructing the underlying components.

For context and usecase, see #125.

[ctz]

I guess the other option would be to make these fields public -- this API also commits us to never adding new fields that need non-trivial values (and I think this is OK for this struct).

[djc]

I guess the other option would be to make these fields public -- this API also commits us to never adding new fields that need non-trivial values (and I think this is OK for this struct).

I'll follow up on that.

[lucab]

I guess the other option would be to make these fields public

Indeed. Two options I considered (and discarded) before opening this PR:

• Changing https://docs.rs/tokio-rustls/latest/tokio_rustls/struct.LazyConfigAcceptor.html#associatedtype.Output to return a tuple of (IO, tls_accepted) components. Not very ergonomic IMHO, and still requires an additional constructor on StartHandshake.
• Making the fields public to allow destructuring StartHandshake. Similar to the current approach, but the two methods here still leave some room to add more private fields in the future (e.g. if they impl Default).