Safe way to enable Discard/ZeroBlocks feature.

[gy2411]

There is a VhostDiscardEnabled feature in the service config https://github.com/ydb-platform/nbs/blob/main/cloud/blockstore/config/server.proto#L243. However, this feature is non-revertible: once a disk starts supporting discards, there is no way to turn that off. We need the safest possible way to enable discards.

Plan:

• Add a VhostDiscardEnabled flag to VolumeConfig in Schemeshard https://github.com/ydb-platform/nbs/blob/main/contrib/ydb/core/protos/blockstore_config.proto#L34. Note that once this flag has been set, it cannot be unset.
• Add a feature to the storage config that will set the VhostDiscardEnabled flag in VolumeConfig when creating new disks. This feature can be enabled for specific clouds/folders, as well as for a percentage of clouds/folders.
• Add a feature to the storage config that will set the VhostDiscardEnabled flag in VolumeConfig for already existing disks. This feature can also be enabled for specific clouds/folders, as well as for a percentage of clouds/folders.

This way, discards can be rolled out gradually.

Approximate order of the feature deployment:

1. Turn on VhostDiscardEnabled in SchemeShard for new volumes on test clouds/folders.
2. Turn on VhostDiscardEnabled in SchemeShard for small persentage of clouds/folders.
3. Turn on VhostDiscardEnabled in SchemeShard for all persentage of clouds/folders.
4. Like 1-3, but for existing volumes.
5. Trun on VhostDiscardEnabled in server config.