create CODEOWNERS (radius-project#8004)

# Description

_Please explain the changes you've made._

Adding a CODEOWNERS file for administering repo membership and roles.

## Type of change

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- This pull request is a minor refactor, code cleanup, test improvement,
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Fixes: N/A

## Contributor checklist
Please verify that the PR meets the following requirements, where
applicable:

- [x] An overview of proposed schema changes is included in a linked
GitHub issue.
- [x] A design document PR is created in the [design-notes
repository](https://github.com/radius-project/design-notes/), if new
APIs are being introduced.
- [x] If applicable, design document has been reviewed and approved by
Radius maintainers/approvers.
- [x] A PR for the [samples
repository](https://github.com/radius-project/samples) is created, if
existing samples are affected by the changes in this PR.
- [x] A PR for the [documentation
repository](https://github.com/radius-project/docs) is created, if the
changes in this PR affect the documentation or any user facing updates
are made.
- [x] A PR for the [recipes
repository](https://github.com/radius-project/recipes) is created, if
existing recipes are affected by the changes in this PR.

Signed-off-by: Will Tsai <28876888+willtsai@users.noreply.github.com>

CODEOWNERS

@@ -0,0 +1,2 @@
+# These owners are the maintainers and approvers of this repo
+*       @radius-project/maintainers-radius @radius-project/approvers-radius