Support user-defined PskId

[arnauorriols]

#237 (comment)

DoD

• Users can define the PskId of the Psk.
• Psk::to_pskid seems to express that PskId are always derived from Psk. To avoid confusion, this method should be renamed. Example: Psk::derive_pskid

Tips

• Psk::to_pskid() is an indicative of the places that assume PskId are always derived from the Psk. Search places where Psk::to_pskid is used, and replace it for explicit PskId value

[kwek20]

As discussed when changing PSK location; Do we really need PskId?

Pros:

• Users can make PskId aliases for easier usage and readability
• No need to externally manage Psk after adding to stream, only PskId

Cons:

• Users need to keep track of PskIds
• Is the Psk realistically only for this stream?