Use custom serialization of rabbitmq role

[cipherboy]

As reported by @mr-miles, the rabbitmq plugin can cause the audit subsystem to crash, due bad response serialization:

2024-03-19T18:50:37.610-0400 [ERROR] TestBackend_RoleReadCrash.core0.audit: panic during logging: request_path=rabbitmq/roles/newrole error="reflect: reflect.Value.Set using unaddressable value"
... output elided ...
  | github.com/openbao/openbao/audit.HashResponse(0xc000739c00, 0xc000ccc6e0, 0x0, {0x0, 0x0, 0x0}, 0x0)
  | \t/home/cipherboy/GitHub/cipherboy/openbao/audit/hashstructure.go:148 +0x311
  | github.com/openbao/openbao/audit.(*AuditFormatter).FormatResponse(0xc000760130, {0x33d9068, 0xc0011be210}, {0x33b2b40, 0xc0011be8a0}, {0x0?, 0x0?, 0x0?, 0x0?}, 0xc000304b60)

This occurs because the output type of the response is bad and includes nested private types (rabbitmq.vhostTopicPermission):

response from structs: map[string]interface{}
 {"tags":"administrator",
  "vhost_topics":map[string]map[string]rabbitmq.vhostTopicPermission
  {"/":map[string]rabbitmq.vhostTopicPermission{
   "amq.topic":rabbitmq.vhostTopicPermission{Write:".*", Read:".*"}}},
  "vhosts":map[string]interface{}{
   "/":map[string]interface{}{
    "configure":".*", "read":".*", "write":".*"}}}

Fixing this requires custom serialization of the response, in line with other plugins (e.g., PKI).

Resolves: #97