Update Offer message type to use ContentKey type

[njgheorghita]

More of a placeholder so I don't lose track of this, but anybody can feel free to pick this one up.

[Narasimha1997]

@njgheorghita
Can you please explain this? I would like to work on this.

[njgheorghita]

Hey 👋 - Basically, we need to update the Offer message type here to use our custom ContentKey type here for its content_keys field instead of Vec<Vec<u8>>

So, the Offer type should look like ..

#[derive(Debug, PartialEq, Clone, Encode, Decode)]
pub struct Offer {
    pub content_keys: Vec<ContentKey>,
}

[KolbyML]

Solved with this PR #325