Minor: Fix Mgmt UI local shovel delete-after

[gomoripeti]

Proposed Changes

Context: Management UI > Shovel Management Tab > "Add a new shovel" section.

There was no input field to set the number for delete-after "After num messages" parameter for local shovels as opposed to AMQP 1.0 shovels.

The help text was also missing for local shovel auto-delete. (The ? showed an empty popup when Local protocol is selected for Source)

The solution is just a copy past from AMQP 1.0

Setting delete-after to a number when creating a local shovel still doesn't work as the value is sent in the PUT json as a string instead of a number. Doing the same with an AMQP 1.0 works fine sending a json number. Where the conversion happens on the frontend is beyond my understanding of the Mgmt javascript. I would need some help to address this part.

{
  "component": "shovel",
  "vhost": "/",
  "name": "s2",
  "value": {
    "src-uri": "amqp://",
    "dest-uri": "amqp://",
    "ack-mode": "on-confirm",
    "src-protocol": "local",
    "dest-protocol": "local",
    "src-queue": "cq1",
    "src-delete-after": "3", <--- string instead of number
    "dest-queue": "cq2",
    "dest-add-forward-headers": false
  }
}

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