Support GitLabs MLFlow backend

[freemin7]

I got it sort of working in GitLab

For that i needed to undo https://github.com/JuliaAI/MLFlowClient.jl/pull/36 locally.

And artifact uploading is currently broken due to it not being supported by the library. I have a patch for that too.

I see two paths forward i could see working:

• Do an AbstractMLFlow of which GitLabMLFlow and DagshubFlow are implementations which diverge in how URI is calculated
• Break how URI is calculated and change the internal structure of MLFlow to allow different endpoints based on arguments passed at the MLFlow struct creation

[pebeto]

Hi.

mlflow contains two endpoints for external communication: /ajax-api and /api. As far as I understand, the /ajax-api only works in the graphical instances of the app, and /api for the REST server. There is a particular case in DagsHub, where the /api endpoint was removed by unknown reasons. I expected the possibility of this issue.

In my opinion, point 2 is a better option. We can add an argument like MLFlow(ajax_endpoint=...).