Adding CODEOWNERS (#8457)

### Description

This adds the CODEOWNERS file to the repo to automatically requests
reviews from people who are responsible for parts of MONAI. This is
currently just the code team but others who have had a hand in specific
parts should be added so that it's clear who could be requested to
comment or review on specialised topics.

### Types of changes
<!--- Put an `x` in all the boxes that apply, and remove the not
applicable items -->
- [x] Non-breaking change (fix or new feature that would not break
existing functionality).
- [ ] Breaking change (fix or new feature that would cause existing
functionality to change).
- [ ] New tests added to cover the changes.
- [ ] Integration tests passed locally by running `./runtests.sh -f -u
--net --coverage`.
- [ ] Quick tests passed locally by running `./runtests.sh --quick
--unittests --disttests`.
- [ ] In-line docstrings updated.
- [ ] Documentation updated, tested `make html` command in the `docs/`
folder.

---------

Signed-off-by: Eric Kerfoot <eric.kerfoot@gmail.com>
Signed-off-by: Eric Kerfoot <17726042+ericspod@users.noreply.github.com>
Co-authored-by: YunLiu <55491388+KumoLiu@users.noreply.github.com>

Author: ericspod

.github/CODEOWNERS

@@ -0,0 +1,17 @@
+/monai/ @KumoLiu @ericspod @Nic-Ma
+/docs/ @KumoLiu @ericspod @Nic-Ma
+/tests/ @KumoLiu @ericspod @Nic-Ma
+/.github/ @KumoLiu
+/monai/networks/schedulers/ @virginiafdez
+/monai/inferers/inferer.py @virginiafdez
+/monai/losses/adversarial_loss.py @virginiafdez
+/monai/losses/perceptual.py @virginiafdez
+/monai/networks/blocks/spade_norm.py @virginiafdez
+/monai/networks/nets/autoencoderkl.py @virginiafdez
+/monai/networks/nets/controlnet.py @virginiafdez
+/monai/networks/nets/diffusion_model_unet.py @virginiafdez
+/monai/networks/nets/patchgan_discriminator.py @virginiafdez
+/monai/networks/nets/spade_autoencoderkl.py @virginiafdez
+/monai/networks/nets/spade_diffusion_model_unet.py @virginiafdez
+/monai/networks/nets/spade_network.py @virginiafdez
+/monai/networks/nets/vqvae.py @virginiafdez