Add type `SnapshotName`

Work done this PR:

Added SnapshotName variable
Ensured all the type casting between SnapshotName and string is done correctly e2dc5b8c766a39431d4ef207116c8f2a9885fc54 9426f5ac84211f98db554c5447a491494b1bcf2a
Added SnapshotName.Validate() 4b6250eabb0f6984e16b16b225682e7fffae3b37
Added test for SnapshotName.Validate() 4b6250eabb0f6984e16b16b225682e7fffae3b37
Added ConfigSnapshot.Validate() dda7c40dc5ff402f4dd50dfb55de03d6c6ac3bf2
Added test for ConfigSnapshot.Validate() dda7c40dc5ff402f4dd50dfb55de03d6c6ac3bf2

Telmate / proxmox-api-go