feat(api) add plugin priority to metadata

[Tieske]

besides exposing the version, the priority of a plugin is also a property that quite often is unclear to users, so dynamically getting it from the system will help answering those questions.

I didn't include it in the original PR (#8810 ), because of the upcoming dynamic priorities. But after a chat with @jschmid1 it turns out that the hard-coded priorities will remain the default, so still makes sense to expose those as well.

Also; removed the true placeholder, which doesn't make sense anymore now that the metadata is an object

[Tieske]

CI failures seem unrelated???

[StarlightIbuki]

I got tests failed earlier today and tried to revert this commit, and it works. This PR seems to be the reason for CI failures?

[StarlightIbuki]

I tried to dump the metadata and found this:

2022/05/20 17:22:10 [error] 14757#0: *5 [lua] kong.lua:96: fn(): {
  ["priority"] = -inf
}, client: 127.0.0.1, server: kong_admin, request: "GET / HTTP/1.1", host: "127.0.0.1:9001"

...

2022/05/20 17:22:09 [error] 14757#0: *4 [lua] api_helpers.lua:511: handle_error(): ...al/kong-dep/luarocks/share/lua/5.1/lapis/application.lua:424: ...kong-dep/luarocks/share/lua/5.1/kong/api/api_helpers.lua:285: body encoding failed while flushing response: Cannot serialise number: must not be NaN or Infinity

It seems that lapis does not support -inf and inf in the meta data.

[Tieske]

@suika-kong my bad. Failures seemed non-related, but unfortunately were. Fix is here: #8833