V2 to v3 updates

[hulkoba]

This work is conducted in accordance with "M2: Add additional RectorPHP rules to upgrade phpseclib 2.0 to 3.0 " as outlined in the Scope of Work for the Sovereign Tech Agency Resilience Program.

This PR

• updates PublicKeyLoader
  • to tweak fluent function to add assign variable to setEncryptionMode and setSignatureMode.
  • keep withSaltLength immutable
  • Add more tests to getLength
• Adds new rule: SymmetricKeyConstructor
  • incl. documentation and tests

It also changes the folder structure to prepare the project for the ruleset for the v3 to v4 update.

The new structure is

rector_rules/ 
- config/
    - v2-to-v3.php
- src/ 
    - Set/ 
        -  V2toV3Set.php 
        -  V3toV4Set.php (coming soon)
    - Rector/
        - V2toV3/ 
            - Rule.php
        - V3toV4/ (coming soon) 
            - Rule.php
- tests/ 
    - V2toV3/
    - V3toV4/ (coming soon)
- composer.json