Define equality for non-trivial data types

[a-sully]

From https://github.com/webmachinelearning/webnn/pull/787/files#r1854479095

We should define [=MLOperandDescriptor/equals=] method to be used when comparing MLOperandDescriptors

[inexorabletash]

And more context (given in the link above) - https://garykac.github.io/procspec/#conditions has good guidance; Infra doesn't define equality for maps, so we should either define it for descriptors, or maps in general (and upstream that).

The spec also does list equality all over the place, e.g. "If its shape is not equal to « outputChannels », then..." and Infra doesn't define equality for lists either. List equality is probably more obvious than map equality.