Add subject mapping transforms for Streams

[Jarema]

With nats-server 2.10.0 this feature will be released.

Server PR:
nats-io/nats-server#3798
PR contains a lot of details.

Example Stream config with transforms:

{
  "name": "test2",
  "subjects": [
    "a",
    "b"
  ],
  "retention": "limits",
  "max_consumers": -1,
  "max_msgs_per_subject": -1,
  "max_msgs": -1,
  "max_bytes": -1,
  "max_age": 0,
  "max_msg_size": -1,
  "storage": "file",
  "discard": "old",
  "num_replicas": 1,
  "duplicate_window": 120000000000,
  "input_subject_transform": {
    "src": "fromtest.>",
    "dest": "input.>"
  },
"sources": [
    {
      "name": "test",
      "subject_transform": "fromtest.>",
      "subject_filter" : ">"
    }
  ],
  "sealed": false,
  "deny_delete": false,
  "deny_purge": false,
  "allow_rollup_hdrs": false,
  "allow_direct": false,
  "mirror_direct": false
}

nats-io/nats-architecture-and-design#201