Stop supporting user-defined error decoders

[whickman]

What happened?

Custom error decoders are able to arbitrarily modify request and response bodies. This produces a huge API surface and a huge potential spread of client behavior. We should remove this ability and become opinionated about error handling.

This blocks moving retry behavior into middleware before error decoding, as retry behavior depends on whether or not an error was returned.

Ideally, we would also remove the WithDisableRestErrors functionality, although that will require more work.

What did you want to happen?

Users should not be able to define custom decoders.

[whickman]

I think we will probably want to move retry logic down and just inform consumers that there's been an API break and their error decoders will no longer trigger retries... This unblocks correct retrying functionality.

It is still useful to get rid of this, though