[Plugin] Custom flags in external plugins not discovered when enabled using kwargs

**Describe the bug**

## DOES NOT WORK
`proxy.Proxy(plugins=["...", "...",])` will not result in discovering custom flags defined in the external plugins.

## FOLLOWING 2 SCENARIO WORKS
`proxy.Plugin(*("--plugin", "...", "--plugin", "..."))`

or

`proxy --plugin ....`

## FIX

`args.plugins` must be resolved prior to calling `Flags.parse` for custom flag discovery.