checkout: Expose --allow-missing arg.

[daavoo]

$ git clone https://github.com/iterative/example-get-started-experiments
$ cd example-get-started-experiments
$ dvc fetch models/model.pkl
$ dvc checkout
A       models/model.pkl
ERROR: Checkout failed for following targets:
data/pool_data
data/train_data
data/test_data
model.tar.gz
models/model.pth
results/evaluate/plots/images
Is your cache up to date?
<https://error.dvc.org/missing-files>

$ git clone https://github.com/iterative/example-get-started-experiments
$ cd example-get-started-experiments
$ dvc fetch models/model.pkl
$ dvc checkout --allow-missing
A       models/model.pkl

[daavoo]

Just noticed, --allow-missing is too silent in the UI when nothing is found, it might be good to at least show the warning with the missing things even if we don't fail

[codecov]

Codecov Report

Patch coverage is 100.00% of modified lines.

Files Changed	Coverage
tests/unit/command/test_checkout.py	ø
dvc/commands/checkout.py	100.00%

📢 Thoughts on this report? Let us know!.

[dberenbaum]

Does it close #9917?

[daavoo]

Does it close #9917?

Not sure, there are other flags requested/suggested but maybe this one is enough