feat(convert): set default model and endpoint variables for services in convert.go

[idsulik]

What I did
This PR resolves issue #13341 by automatically setting default model_var and endpoint_var values for services that use models but don't explicitly define these variables

Related issue
#13341

[ndeloof]

default values should be set by compose-go as we load a project IMHO

[idsulik]

@ndeloof but I'm not sure if it ok to make these changes for all places, because the documentation says only about environment values https://docs.docker.com/reference/compose-file/services/#models

If either is omitted, Compose automatically generates the environment variable names based on the model key using the following rules...

but in this PR I made the changes only for the converter, because it doesn't work even if you specify the environment values