add Backend Protocol to aid backend developers

[aaronzo]

based on discussion in #7334. This adds a backend protocol that devs can use to varying degrees to ensure their backend conforms to networkx's contract.

How does this help backend developers?

Easiest to give examples of various patterns and options this enables. All of these snippets would live in backend codebases, and assume the import

from networkx.utils.interface import BackendInterface

1. No inheritance, just instance checks

class Dispatcher: 
    ...

def test_dispatcher_interface():
    # check all the methods networkx can call are implemented
    assert isinstance(Dispatcher, BackendInterface)

2. With inheritance

class Dispatcher(BackendInterface[ga.Graph]):  # optionally specify backend graph type
    # checks method existence upon class definition
    # IDE autocompletes functions with correct signatures for backend, and have information for static type checkers
    ...

3. With inheritance and plugin entry point check

class Dispatcher(BackendInterface, plugin=True): 
    # checks method existence upon class definition
    # checks that this class is the value for an entry point in the "networkx.backends" group
    ...

4. With inheritance, plugin entry point check and get_info check

class Dispatcher(BackendInterface, plugin=True, info=True):
    # checks method existence upon class definition
    # checks that this class is the value for an entry point in the "networkx.backends" group
    # checks that a `get_info` function is given as an entry point
    # checks that info given for the plugin is consistent with this class
    # checks that this backend implements networkx-registered algorithms
    ...

5. With inheritance and fallback function

class Dispatcher(BackendInterface, fallback=...):
    # checks method existence upon class definition
    # provides a fallback whenever this class doesn't have an attribute or method (like in nx-loopback)
    ...

fyi @eriknw @rossbar