fix: add name field in plugin_config schema for consistency

[Revolyssup]

Description

Fixes apache/apisix-ingress-controller#2003
All core resources like service, upstream , route, plugin have name field but plugin_config doesn't and after this PR where additional fields were prohibited, passing name in plugin_config gives an error. Ingress controller currently doesn't differentiate and adds name to plugin_config as well so currently there is this issue between apisix and ingress controller.

So this PR adds the name field to plugin_config to solve the compatibility issue and also for consistency

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)