Add ability to use different Objectdetection models than FasterRCNN

## 🚀 Feature
It would be good to be able to use more up to date models than FasterRCNN in raster-vision.
Torchvision offers some possible dropin replacements / alternatives like RetinaNet or SSD based detectors.

## Motivation
More up to date models promise better performance at lower GPU needs.

## Pitch

The object_detection_learner in raster-vision should be able to accept a configuration parameter which allows a different model to be used, as written above.

## Alternatives

The user could do this manually in the code, but this is not very scaleable and unclear how many manual changes are needed.

## Additional context

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Start at the object_detection_learner entry point and review how its model is currently selected. Compare the requested RetinaNet or SSD alternatives and define a configuration path that lets users choose them; done means the learner accepts that choice and supports the selected detector.

Written by the indexing model from the issue text.