[Feature Request] Allow passing variables when creating releases

**Is your feature request related to a problem? Please describe.**
I cannot pass override values for release variables when I am creating a release.

**Describe the solution you'd like**
Allow the `az pipelines release create` command to accept a variables parameter of key/value pairs, basically exactly the same way it is used in `az pipelines build queue`.

eg. `az pipelines release create --definition-id 1 --variables var1=foo var2=bar`