Adds two new endpoints to the pluginhub portion of the API which can be used with https://shields.io to generate README badges for Github pages. These endpoints would be used with the Endpoint Badges portion of the shields.io website (https://shields.io/badges/endpoint-badge)

I'm not sure if this is actually feasible within this API as the runlite-{version} part will change with every release but I figured I'd PR it anyway. Maybe the API can support latest-release as a version to support this PR? If not feel free to close this.

1. /shields/installs/plugin/{pluginName}
2. /shields/rank/plugin/{pluginName}

Example shields.io format (assuming latest-release would work):
https://img.shields.io/endpoint?url=https://api.runelite.net/latest-release/pluginhub/shields/installs/plugin/quest-helper

I copied these URL paths from my personal website, if they are too verbose let me know and I can change them.

I was able to test this functionality through IntelliJ Ultimate Edition. I used the below generated-requests.http file to populate dummy data:

###
POST http://localhost:8080/pluginhub
Content-Type: application/json
X-Forwarded-For: 127.0.0.1

["test", "test1"]

###
POST http://localhost:8080/pluginhub
Content-Type: application/json
X-Forwarded-For: 127.0.0.2

["test", "test2"]

###
POST http://localhost:8080/pluginhub
Content-Type: application/json
X-Forwarded-For: 127.0.0.3

["test", "test3"]

###
POST http://localhost:8080/pluginhub
Content-Type: application/json
X-Forwarded-For: 127.0.0.4

["test", "test4"]

###
POST http://localhost:8080/pluginhub
Content-Type: application/json
X-Forwarded-For: 127.0.0.5

["test", "test5"]

pluginhub endpoint (unchanged)

plugin value exists:

plugin value does not exist: