Add identity info to activity export API

[ccapurso]

Description

RFC: [VLT-324 Enhance Activity Export API with Identity Metadata](https://go.hashi.co/rfc/vlt-324)

This PR expands on #27846 and https://github.com/hashicorp/vault-enterprise/pull/6171 to enhance the activity export API to include identity metadata. The following overall changes are included:

• VAULT-29101: Add identity metadata fields to json format.
• VAULT-29102: Add identity metadata fields to csv format. Map and slice fields are flattened into individual columns.
• No data found within requested start_time and end_time window now results in 204 No Content instead of 400 Bad Request.
• A new semgrep pattern for the nil-check-physical-storage-by-nsid rule

ENT PR: https://github.com/hashicorp/vault-enterprise/pull/6287

TODO only if you're a HashiCorp employee

• Labels: If this PR is the CE portion of an ENT change, and that ENT change is
  getting backported to N-2, use the new style backport/ent/x.x.x+ent labels
  instead of the old style backport/x.x.x labels.
• Labels: If this PR is a CE only change, it can only be backported to N, so use
  the normal backport/x.x.x label (there should be only 1).
• 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! ✅

[divyaac]

Looks great!

[akshya96]

LGTM!