Add _initializableStorageSlot

[ernestognw]

Fixes #4782

Adding _initializableStorageSlot seems like a good abstraction for developers who want to override the initializable storage slot. This makes sense in the context of Account Abstraction since some wallets might reuse the slot. For these case it may be better that each implementation has its own customized slot.

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• Tests
• Documentation
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openzeppelin-solidity	Minor

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[ernestognw]

Alternatively, we can make _getInitializableStorage internal. Thoughts?

[Amxx]

I'm concerned by line 63

     * when using with upgradeable contracts.
     *
     * @custom:storage-location erc7201:openzeppelin.storage.Initializable // <<== HERE !
     */
    struct InitializableStorage {

This comment will be wrong if the value is overriden. How will the plugin handle that ?

[Amxx]

function _getInitializableStorage() internal virtual pure returns (InitializableStorage storage $) {

Would be a smaller change, that we could easily generalize to all the upgradeable contracts. The downside is that overriding it would likely require the user writting assembly.

[arr00]

function _getInitializableStorage() internal virtual pure returns (InitializableStorage storage $) {

Would be a smaller change, that we could easily generalize to all the upgradeable contracts. The downside is that overriding it would likely require the user writting assembly.

I imagine that most teams requiring this functionality won't have an issue writing minimal yul. Don't think it's worth the extra function call just to bypass that.