VAULT-31409: trace postUnseal function (#28895)

* initial implementation of unseal trace

* close file if we fail to start the trace

didn't bother to check the error from traceFile.Close()

* use reloadable config instead of env var

* license

* remove leftover

* allow setting custom dir and remove new package

* bring back StartDebugTrace

after talking to Kuba it sounds like it's a good idea to try to move stuff out of core, so even if there's no immediate need for a generic debug trace function it's still fair to add it

* track postUnseal instead of unsealInternal

also some usability improvements from manual testing

* address PR comments

* address security review

there were concerns about using the /tmp directory because of permissions, or having a default dir at all, so now it's required to set a dir in order to generate the traces.

* add unit tests to StartDebugTrace

* move back to default dir

* document new parameters

* add tiny integration test

* avoid column in trace filename

sounds like it might be forbidden in Windows and possibly cause problems in some MacOS applications.

* address PR feedback

* add go doc to test

CI was complaining about missing comments on the new test function. It feels a bit silly to require this of tests but whatever XD

* fix tests

command/server/config_test_helpers.go

@@ -854,6 +854,8 @@ func testConfig_Sanitized(t *testing.T) {
 		},
 		"administrative_namespace_path": "admin/",
 		"imprecise_lease_role_tracking": false,
+		"enable_post_unseal_trace":      true,
+		"post_unseal_trace_directory":   "/tmp",
 	}
 
 	addExpectedEntSanitizedConfig(expected, []string{"http"})