add scope to plugin attributes

[tzssangglass]

Now we have a lot of plugins which have different scopes. We do not maintain a correspondence between plugins and scopes.So users may be confused as to why a plugin cannot be enabled on route.

Here are some examples:

• batch-requests cannot be used on routes
• jwt-auth cannot be used in routing alone, must be combined with consumer
• error-log-logger can only be used at instance level

So I want to add scope attributes to all existing plugins to indicate at what level they can be used.

The scope enums like route, service, consumer, global.

Here are some clarifications

the scope of a plugin can have multiple values, sucn as scope = { "route", "global" }
global is the scope for plugins and the existing global_rule is the concept for APISIX, they do not contradict each other. For example, if the scope attribute of a plugin is global, then the plugin can only be configured in global_rule.

What are your options? @spacewander @liuxiran @membphis @juzhiyuan

The discussion here is only supplementary and I will start this discussion on the mailing list later.

[tzssangglass]

related to: apache/apisix-dashboard#2110