crypto/acme: add RetryAfter field to the Authorization, Order & Challenge objects

[ThatsMrTalbot]

When using WaitAuthorization or WaitOrder is not practical due to its blocking,
for example in Kubernetes controllers, the GetAuthorization can be used.
This change adds the RetryAfter field to the Authorization, Order and Challenge
objects returned by GetAuthorization, GetOrder and GetChallenge so it can be
used by these implementations that use their own polling mechanism.

The new RetryAfter field is populated by the "Retry-After" header in the HTTP
response.

Fixes golang/go#74454

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Patch Set 1:

(1 comment)

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Message from Daniel McCarney:

Patch Set 2:

(1 comment)

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Message from Adam Talbot:

Patch Set 3:

(2 comments)

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This PR (HEAD: 061aeaf) has been imported to Gerrit for code review.

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