Handle MongoDB 4.2 in DDS resource

[Karreg]

For now, MongoDB 4.2 is not supported by the provider.
I see at least two changes:

• Allow MongoDB 4.2 version
• Allow RocksDB as storage engine

terraform-provider-flexibleengine/flexibleengine/resource_flexibleengine_dds_instance_v3.go

Lines 57 to 71 in 1dfbbf4

	"version": {
	Type: schema.TypeString,
	Required: true,
	ForceNew: true,
	ValidateFunc: validation.StringInSlice([]string{
	"4.0", "3.4",
	}, true),
	},
	"storage_engine": {
	Type: schema.TypeString,
	Optional: true,
	ForceNew: true,
	ValidateFunc: validation.StringInSlice([]string{
	"wiredTiger",
	}, true),

Thanks!