[FEATURE REQUEST] oiiotool op graph introspection

Following from a discussion on [slack](https://academysoftwarefdn.slack.com/archives/C05782U3806), there was a desire for `oiiotool` to be able to modify/supply parameters to its own `ImageInput` readers based on the connected `ImageOutput` writers.

At present `oiiotool` parses its arguments in order of declaration, so by the time the output declarations are reached, all the `ImageInput`'s are already instanced.
The workaround is currently to use `--iconfig` to manually specify any desired changes to the ImageInput, but there may be cases where we would like these changes to happen "automagically".

Use case could include:
* Avoiding lossy conversions when round tripping image formats
* Minimizing data type conversions (eg uint8->float->uint8)
* Allowing inter-op communication

A simple implementation might just be to parse the ops into a graph prior to instantiation, and write a small query API to allow introspection.

Start by tracing how oiiotool parses declarations and instantiates ImageInput readers and ImageOutput writers, comparing that flow with the existing --iconfig workaround. Define the operation graph and query API needed for output settings to influence inputs, then validate the design against lossy conversions, data type conversions, and inter-op communication.

Written by the indexing model from the issue text.