Update our Lua bridge resolver to use the new Kong DNS resolver

[thibaultcha]

See the new DNS resolver library enabled on-demand in Kong 3.x series: Kong/kong#12305

And our invocation of the Kong resolver from Wasm filters: https://github.com/Kong/ngx_wasm_module/blob/prerelease-0.4.0/src/common/lua/ngx_wasm_lua_resolver.c#L45-L71

We will need to update our Lua code to use the appropriate resolver depending on Kong's configuration.

[flrgh]

Haven't looked closely at the new DNS resolver yet; I'm guessing some of its API has diverged from resty.dns.client?

If so, would it be more appropriate for us to handle this within Kong gateway by putting a compat shim around the new DNS resolver? This way ngx_wasm_module can remain fully compatible with resty.dns.client?

[thibaultcha]

Thanks for looking into it! Either way sounds good to me...

[flrgh]

relevant: Kong/kong#13725

The gateway now has test coverage for both new/old DNS client implementations. Under the hood the test uses /etc/hosts style hardcoding of name->ip relationship, so it's not a very deep test and won't catch things like yielding differences between the two clients, but it will certainly catch any problems that would occur from differences in the Lua API.