Backport of Support mlock and custom tmpdir for containerized plugins into release/1.15.x

[hc-github-team-secure-vault-core]

Backport

This PR is auto-generated from #23215 to be assessed for backporting due to the inclusion of the label backport/1.15.x.

🚨

Warning automatic cherry-pick of commits failed. If the first commit failed,
you will see a blank no-op commit below. If at least one commit succeeded, you
will see the cherry-picked commits up to, not including, the commit where
the merge conflict occurred.

The person who merged in the original PR is:
@tomhjp
This person should manually cherry-pick the original PR into a new backport PR,
and close this one when the manual backport PR is merged in.

merge conflict error: POST https://api.github.com/repos/hashicorp/vault/merges: 409 Merge conflict []

The below text is copied from the body of the original PR.

---

When running containerised plugins within the context of our default systemd unit file (from the rpm repositories), I found a couple of issues I hadn't stumbled on previously:

• Plugins need a capability to mlock if Vault is configured with mlock - previously they always inherited this from Vault as a sub-process, but now we need to explicitly set it for containers. It boils down to adding the IPC_LOCK capability to the container. Without it, we get an error on plugin startup because the mlock syscall fails. Library PR: plugincontainer: Support mlock go-secure-stdlib#94
• If running with systemd's PrivateTmp setting, the container cannot see Vault's view of /tmp, so we need the ability to negotiate a separate directory that both sides can see. We could just use TMPDIR, but then that ruins the whole point of the setting. Instead, we introduce a new environment variable to use a different tmp dir in one very narrow use-case. Library PR: Add TempDir option to UnixSocketConfig go-plugin#282

---

Overview of commits

• a074bf9

[hashicorp-cla]

Thank you for your submission! We require that all contributors sign our Contributor License Agreement ("CLA") before we can accept the contribution. Read and sign the agreement

Learn more about why HashiCorp requires a CLA and what the CLA includes

---

temp seems not to be a GitHub user.
You need a GitHub account to be able to sign the CLA. If you already have a GitHub account, please add the email address used for this commit to your account.

_{Have you signed the CLA already but the status is still pending? Recheck it.}

[github-actions]

CI Results:
All Go tests succeeded! ✅