Refactor the crypto wallet model

[aurelticot]

As of creating this ticket, the crypto wallets are stored and managed in the app with an object structured by blockchain:

{
  "eip155:5": {
    "address": "0xABCD...",
    "...": "..."
  },
  "eip155:80001": {
    "address": "0xABCD...",
    "...": "..."
  },
  "near:testnet": {
    "address": "A1B2C3...",
    "...": "..."
  },
}

For a given blockchain typology (eip155), we are duplicating the information for each chain (5, 80001, etc).
If we dynamically add a new network from Wallet Provider, we'd need some logic to create the corresponding "wallet" for this chain as well.

We should instead only organise the wallet by blockchain typology and not care about the actual chain/network.

Actual model and schema to be defined, but something like this:

[{
  "namespace": "eip155",
  "address": "0xABCD...",
  "...": "..."
}, {
  "namespace": "near",
  "address": "A1B2C3...",
  "...": "..."
}]

Note: Changing the persistence schema will require a migration logic in the wallet.

[aurelticot]

Related to #1089

[aurelticot]

Once done, optimise the app startup by storing the crypto wallet in the cache or local secure store rather than fetching them from the user's Verida Vault (still have to fetch them but in the background, for update, not as a sequential step at the start).

It used to be the case and worked but had to refetch from the Vault due to the enabling of other networks.