Audit: always use derived context for audit logging, increase timeout to 10s

[peteski22]

Description

This PR makes the following changes to the Audit system:

• Always use a newly derived context so that we cannot be cancelled by the HTTP request etc.
• Increase new context timeout to 10 secs
• Remove non-required code and tests (isContextViable)

No ENT PR required.

NOTE:

The reason we are not currently looking into making the timeout tuneable/configurable is that it's an easy/surefire way to brick your Vault cluster if you turn this up too high.

Each attempt to log a request or response means the eventlogger could spin up multiple goroutines for Vault's audit devices, if we're dealing with clusters experiencing 10s of thousands of requests per second, this could mean there's a lot 'in flight', and giving audit longer and longer likely won't resolve an issue if a file/socket/syslog cannot write within ~10secs.

It just means we would be making the eventlogger wait longer and longer before deciding if we wrote to enough audit sinks. With Vault sat waiting in the meantime.

HashiCorp employee checklist

• Backport Labels: If this PR is in the ENT repo and needs to be backported, backport
  to N, N-1, and N-2, using the backport/ent/x.x.x+ent labels. If this PR is in the CE repo, you should only backport to N, using the backport/x.x.x label, not the enterprise labels.
  • 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.

[github-actions]

CI Results:
All Go tests succeeded! ✅

[github-actions]

Build Results:
All builds succeeded! ✅

[VioletHynes]

Looks great! I love this idea, and it's a nice concise implementation with good tests :)

[kubawi]

LGTM