[Feature]: Make inner fields private for our contracts

[qalisander]

What is the feature you would like to see?

Currently our contracts expose inner fields as publicly accessible and looks like that:

/// State of an [`Erc721`] token.
#[storage]
pub struct Erc721 {
    /// Maps tokens to owners.
    pub _owners: StorageMap<U256, StorageAddress>,
    /// Maps users to balances.
    pub _balances: StorageMap<Address, StorageU256>,
    /// Maps tokens to approvals.
    pub _token_approvals: StorageMap<U256, StorageAddress>,
    /// Maps owners to a mapping of operator approvals.
    pub _operator_approvals:
        StorageMap<Address, StorageMap<Address, StorageBool>>,
}

Since some of our extensions and example consumers depends on pub availability of inner fields.
We should refactor some tests and examples, to make fields private.
e.g. add pub accessor method should be more preferable, then have a publicly available field.

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