fix: provide device auth config (#8419)

# Which Problems Are Solved

There was no default configuration for `DeviceAuth`, which makes it
impossible to override by environment variables.
Additionally, a custom `CharAmount` value would overwrite also the
`DashInterval`.

# How the Problems Are Solved

- added to defaults.yaml
- fixed customization

# Additional Changes

None.

# Additional Context

- noticed during a customer request

internal/api/oidc/device_auth.go

@@ -60,7 +60,7 @@ func (c *DeviceAuthorizationConfig) toOPConfig() op.DeviceAuthorizationConfig {
 		out.UserCode.CharAmount = c.UserCode.CharAmount
 	}
 	if c.UserCode.DashInterval != 0 {
-		out.UserCode.DashInterval = c.UserCode.CharAmount
+		out.UserCode.DashInterval = c.UserCode.DashInterval
 	}
 	return out
 }