Add a "neighbor" method to `interpolate_bads` that works without exact sensor positions

Describe the new feature or enhancement

interpolate_bads has several methods implemented, like the most commonly used spherical spline approach, but also "mne", and "nearest" (for fnirs, although it is not documented what this entails).

All of these methods require 3D sensor positions to work, however, sometimes these may not be available.

As an alternative, it would be great to pass an adjacency matrix (see e.g., find_ch_adjacency), and have the interpolation be done as a (weighted) average of the neighbors.

This is something that is already supported in FieldTrip: https://github.com/fieldtrip/fieldtrip/blob/55e8f26facac8eaf16ba7222433eba3080803b5c/ft_channelrepair.m#L3-L7

Describe your proposed implementation

1. There should be a function to easily create an "mne adjacency matrix" from scratch, for example by passing a dict of neighbors:

mydict = {
    ch1: [ch2, ch3],  # mapping a channel to its neighbors
    ...
}
adjacency = mne.channels.make_adjacency(mydict)

2. .interpolate_bads method should have a new method argument called neighbors and a new parameter adjacency, which gets used for neighbor interpolation.

Describe possible alternatives

I am not aware of an alternative for channel interpolation when no sensor positions are available.

Additional context

This proposed functionality would have to exhibit some "smart" behavior for cases where too few "good" neighbors are available. Potentially with an additional parameter n_required_good_neighbors (or similar), and if that threshold is not met, no interpolation happens for the channel in question

Start by reading the existing interpolate_bads methods and the find_ch_adjacency entry point. Define how mne.channels.make_adjacency should build an adjacency matrix, how the neighbors method uses adjacency, and what happens when too few good neighbors exist. Done means adjacency-based interpolation works without exact sensor positions and its edge cases are specified.