Allow plugin developers to add custom kong commands

[roskh]

It would be useful for plugin developers to have a way to inject their own custom commands.

In our use-case we developed a custom plugin for fetching a JWKS from a remote URL, which we then use to validate externally issued JWTs.
For this use-case we want to have a command like verify_jwks to ensure that a URL is reachable by a running kong instance.

We've managed to monkey-patch the command (probably not ideal).
Example Dockerfile:

FROM kong:3.9.1-ubuntu

ADD kong-plugin-custom/kong/cmd/verify_jwks.lua /usr/local/share/lua/5.1/kong/cmd/verify_jwks.lua
# monkey patch
RUN sed -i '/check = true,/a verify_jwks = true,' /usr/local/bin/kong 

It then allows us to invoke the command like so:

kong verify_jwks https://token.actions.githubusercontent.com/.well-known/jwks
# exits with 0 or 1 depending on validation success

Following this strategy we've run into some issues with initialising the Kong context/configuration, namely to have our custom command pick up these directives:

KONG_LUA_SSL_TRUSTED_CERTIFICATE
KONG_LUA_SSL_VERIFY_DEPTH

for the rest http client

local http = require "resty.http"
local client = http.new()
local res, err = client:request_uri(jwks_uri, http_options)

Any suggestions where to look would be greatly appreciated, even if all this is very hacky.

PS: let us know if you'd like us to upstream our work on JWKs fetching/validation into the jwt plugin