Expose max API version

[lucacome]

I want to be able to programmatically get the latest API version that the client can use.

For example, if I use NGINX Plus R29 that supports up to version 8 of the API, I want this function to return 8 even if the client supports up to version 9.

If hypothetically R32 comes out with a new version 10 and the client is not yet updated to support it, this function should return 9.