exp init: initialize dvc repo if uninitialized

[skshetry]

Also autostages dvc.yaml/.gitignore/param files by default.

The whole solution feels a bit hacky, as we need to create an uninitialized dvc repo, create a stage in memory and initialize dvc repo if there is no .dvc directory. The "creating stage" part does not really work without creating a proper DVC repo, so we have to hack it around.
For now, removing the RemoteCacheRequired exception throwing does seem to make it work, but it's hard to guarantee that as we have never supported that before.

Part of #6446.

[dberenbaum]

So if the command fails or the user decides not to keep the stage, is the dvc repo initialized? Would it be better to add a hint to dvc init when running a command outside of a dvc repo?

[daavoo]

So if the command fails or the user decides not to keep the stage, is the dvc repo initialized? Would it be better to add a hint to dvc init when running a command outside of a dvc repo?

Upvoting for hint + docs updates

[skshetry]

Closing for now, some of the pre-requisite were backported. It's a bit tricky considering we depend on Repo for so many things, and can be opened again if we think it's important to have.

[skshetry]

So if the command fails or the user decides not to keep the stage, is the dvc repo initialized? Would it be better to add a hint to dvc init when running a command outside of a dvc repo?

No, this PR initialized repo at the very end just before creating dvc.yaml file, but after getting confirmation from the user for stage.