`cirq.plot_decompose(op_tree)` to plot the decompose relationships b/w operations

[tanujkhattar]

As part of https://tinyurl.com/cirq-organize-decompose-rfc, we want to have a cirq.plot_decompose(op_tree) method which can take in an op-tree and plot the decomposition graph built by recursively calling cirq.deocmpose_once() to each operation till they cannot decompose anymore.

Some things to consider:

• Control by a flag whether the decomposition graph should contain individual operations as nodes (eg: cirq.X vs cirq.X ** 0.5) or whether it should contain the operation class as nodes (so both cirq.X and cirq.X ** 0.5 will be represented by XPowGate).
• We can also consider returning the constructed nx.Graph from the method so the user has the opportunity to play around with it if they wish to.

For a reference of the resulting plots, see Visualizing Gate Relationships colab

Part of #3242

P1 - I need this no later than the next release (end of quarter)

[mhucka]

Discussed with @tanujkhattar and @mpharrigan offline: this can be closed, because soon there will be a Qualtran web app that provides some of the functionality discussed here.