Export SQL database to a local file

**Related command**
az sql db export

**Is your feature request related to a problem? Please describe.**
Need an ability to export database into a local `bacpac` file.

**Describe the solution you'd like**
An option to use the `az sql db export` command with local file support. For instance:
```bash
az sql db export \
--resource-group "..." \
--server "..." \
--name "..." \
--admin-user "..." \
--admin-password "..." \
--file "EXPORT_FILE.bacpac" \
```

**Describe alternatives you've considered**
Current alternative solution is to use the `SqlPackage` tool. For instance:
```bash
sqlpackage /a:Export /tf:"EXPORT_FILE.bacpac" /scs:"DATABASE_CONNECTION"
```