Add `inner` method to `BindingsArray`

[chriseclectic]

What should we add?

Add an inner method to BindingsArray that allows combining two bindings arrays with the same shape

Eg

class BindingsArray:

    def inner(self, other: BindingsArray) -> BindingsArray:
         # validate self and other have distinct parameters
         # validate self and other have same shape (or more generally broadcast compatible shapes?)
         # construct the data dict combining the two bas
    ... 

This method could either require the arrays have the same shape, or alternatively that they have broadcastable shapes (eg if other.shape = (*extra_dims, *self.shape) the return would be other.shape, or if other.shape = shape + self.ndim * (1,) the return would be equivalent to the outer product (up to ordering of dimensions). Maybe this method could be called combine or something like that instead.

Related to #12437

[arujjval]

@chriseclectic can I work on this issue?