Add optional service methods

[hlb8122]

Motivation

#265

Solution

This is currently an incomplete solution - it is the minimum required to demonstrate this approach and allow for a decision.

It mirrors closely the way optional_layer method works.

Considerations

• Is this zero-cost?
• Should NamedService be implemented on Either like this? Does this break anything?
• There's an Unimplemented service which seems to do the same thing as EmptyService, perhaps we should use that instead?

TODO

After deciding whether this approach is feasible the following needs to be done:

• Implement add_optional_service onto the Router in a similar manner
• Documentation

[LucioFranco]

Okay, almost there!

[LucioFranco]

🔥