Expose `params`/ `ParamsDependency` in the Python API.

[daavoo]

params are easy to include in a pipeline via templating but using them inside a python script might not be that trivial and require several changes in the user's code (i.e. when a stage uses several params from different params files).

I think that it would be nice to expose some function in the public API that would take care of loading the params associated to a stage and returning them as python objects, directly usable in the python script.

It could be something like dvc.api.get_params() and would take care of loading only the params defined in the params section of the stage.

[daavoo]

For reference, open source project implementing an API similar to what it's described in the issue https://docs.orchest.io/en/stable/fundamentals/sdk/index.html#api-parameters

[jorgeorpinel]

defined in the params section of the stage

How would DVC know what stage this Python file is associated with? Would it only work automatically if the file is in the cmd field of dvc.yaml? If you have to specify which stage then it's less "magical".

UPDATE: Answer in #7613