* add service attachment support to lb app int module
* allow direct referencing of self managed ig in ilb module
* add service attachment support to net-ilb-int
* add service attachments example to net-lb-int
* fix resource name in net-lb-ext
* rename fwd rules resource in test inventories
* add toc to net-lb-int
## net-lb-int
* Fix error on apply of example:
```
Error creating RegionBackendService: googleapi: Error 400: Invalid value for field 'resource.backends[0].balancingMode': 'UTILIZATION'. Balancing mode must be CONNECTION for an INTERNAL backend service., invalid
```
* remove unused `balancing_mode` variable, as only one value is possible anyhow
## net-lb-ext
* update in the `backends` description
## net-lb-proxy-int
* update in the `backends` description
## net-lb-app-int
* added validation of `balancing_mode`
* fixed other validations
## net-lb-app-ext
* added validation of `balancing_mode`
* fixed other validations
* removed validation for `locality_lb_policy` as this variable is not used in this module
Closes: #1767
Ludovico Magnocavallo
Wiktor Niesiobędzki
Luca Prete
Miren Esnaola