Change annealing quadratic keys to strings

[speller26]

Because JSON doesn't support tuple keys

[kshitijc]

I like the version with the dict better since it:

• Implicitly enforces duplicate definitions (defining the same tuple twice).
• Preserves typing information.
• Avoids having to deal with issues like differentiating "1,2" / '1, 2".

[avawang1]

In regards to benefits regarding the string approach:

• The adjacency list approach using the dict leaves open the question of having only some of the nodes defined or having all of the nodes defined (which is basically defining the same tuple twice).
• list of edges for representing quadratic coefficients is personally more intuitive to me than adjacency lists of these coefficients
• when customer does .to_ir() in the SDK, it's easier for them to eyeball and validate
• avoids having to do extra computation from a list of edges to adjacency lists and vice versa
• we expect that customers will have an automated way of writing the IR, so having a string format for the tuples is a bit better