VAULT-36112: Better handling for Retry-After rate limit header.

[miagilepner]

Description

This PR fixes two issues:

1. The api.Client doesn't respect the Retry-After header by default. The PR updates the backoff strategy to use the value from the Retry-After header, if it exists.
2. The vault server rounds down it's retry time to the nearest second. This is particularly bad if the value is 0<x<1 second, because the client will retry immediately and get rejected. This PR updates our logic to round up.

The go-retryablehttp change will need to be merged. @katbyte is the maintainer for that repo and is waiting to check with a team member next week before approving the PR.

TODO only if you're a HashiCorp employee

• Backport Labels: If this fix needs to be backported, use the appropriate backport/ label that matches the desired release branch. Note that in the CE repo, the latest release branch will look like backport/x.x.x, but older release branches will be backport/ent/x.x.x+ent.
  • LTS: If this fixes a critical security vulnerability or severity 1 bug, it will also need to be backported to the current LTS versions of Vault. To ensure this, use all available enterprise labels.
• ENT Breakage: If this PR either 1) removes a public function OR 2) changes the signature
  of a public function, even if that change is in a CE file, double check that
  applying the patch for this PR to the ENT repo and running tests doesn't
  break any tests. Sometimes ENT only tests rely on public functions in CE
  files.
• Jira: If this change has an associated Jira, it's referenced either
  in the PR description, commit message, or branch name.
• RFC: If this change has an associated RFC, please link it in the description.
• ENT PR: If this change has an associated ENT PR, please link it in the
  description. Also, make sure the changelog is in this PR, not in your ENT PR.

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CI Results:
All Go tests succeeded! ✅

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