rusk-wallet: Refactor Wallet struct to make impossible states impossible

[welf]

Currently, our Wallet struct allows for an inconsistent state:

pub struct Wallet<F: SecureWalletFile + Debug> {
    ...
    file: Option<F>,
    file_version: Option<DatFileVersion>,
}

This structure allows for an inconsistent state where file could be None while file_version is Some(_), or vice versa. This not only introduces potential errors but also complicates the code, requiring unnecessary unwrapping of both file and file_version even when one has already been successfully unwrapped. Consequently, the code becomes more verbose and harder to understand and maintain.

A potential solution is to combine both values into a single data structure:

pub struct Wallet<F: SecureWalletFile + Debug> {
    ...
    file: Option<(F, DatFileVersion)>,
}

or

pub struct FileData<F: SecureWalletFile + Debug> {
    file: F,
    file_version: DatFileVersion
}

pub struct Wallet<F: SecureWalletFile + Debug> {
    ...
    file: Option<FileData<F>>,
}

Expected behaviour
The design of the Wallet struct should prevent impossible state.