Refactor: Improve project structure and refactor registerIdentity

[rwixy]

Description

This pull request refactors esx_identity by modularizing the codebase, improving the structure, and applying the DRY principle.
The esx_identity:registerIdentity callback has been cleaned up and optimized, and related logic has been moved into dedicated files for better maintainability.

PR Checklist

• My commit messages and PR title follow the Conventional Commits standard.
• My changes have been tested locally and function as expected.
• My PR does not introduce any breaking changes.
• I have provided a clear explanation of what my PR does, including the reasoning behind the changes and any relevant context.

[ASTROWwwW]

Nice cleanup — the modular split makes esx_identity much easier to follow. Two things worth a look:

1. completeRegistration now fires esx_identity:completedRegistration in the standalone path too. In the current main.lua, esx_identity:completedRegistration is triggered only in the multichar branch (xPlayer == nil). Here Callback.RegisterIdentity calls the shared completeRegistration() in both branches, so a normal non-multichar registration now also emits that event. Anything subscribed to it (esx_multicharacter, third-party resources) will start receiving it in standalone mode — intended, or should completeRegistration skip the event in the xPlayer branch to keep the previous behavior?

2. Two existing gaps you could close while you're in this code:

• validateRegistrationData reads data.firstname directly, so a client triggering esx_identity:registerIdentity with a non-table data throws and aborts the callback — a type(data) ~= 'table' guard at the top of Callback.RegisterIdentity covers it.
• CheckSexFormat accepts "M"/"F" but getFormattedIdentity stores data.sex raw, so "M" gets persisted and breaks downstream sex == "m" checks; lowercasing before save fixes it.

(I have those two in #1787 against main — happy to fold them into validation.lua/callback.lua here instead.)

Minor: registry.lua makes Modules a global; a local returned/exported from the module would avoid the global and the implicit fxmanifest load-order coupling.