Move retry backoff behavior into separate middleware

[advayakrishna]

Before this PR

It's difficult to reason about and modify URI selection or retry behavior because the functionality has been coupled in the RequestRetrier. RequestRetrier as currently implemented is responsible for several things:

• Determining if a response can be retried
• Choosing the URI for the next attempt
• Implementing backoff behavior

This change simplifies backoff behavior and separates it into an independent middleware to begin cleaning this up.

After this PR

==COMMIT_MSG==
Move retry backoff behavior into separate middleware
==COMMIT_MSG==

Possible downsides?

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Type

• Feature
• Improvement
• Fix
• Break
• Deprecation
• Manual task
• Migration

Description

Move retry backoff behavior into separate middleware

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[bmoylan]

Reviewed 4 of 7 files at r1, 3 of 3 files at r2, 1 of 1 files at r3, all commit messages.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @advayakrishna)

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conjure-go-client/httpclient/client.go line 201 at r3 (raw file):

	// call Next once so that the first repeated URI has backoff
	retrier.Next()
	return internal.NewBackoffMiddleware(func() {

nit: internal.NewBackoffMiddleware(retrier.Next)

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conjure-go-client/httpclient/internal/request_retrier.go line 108 at r3 (raw file):

}

func isNonRetryableClientError(resp *http.Response, errCode int) bool {

nit: should this and isSuccess be in the neighboring retry.go file?