Add option to use KDBX3 format

[luckyrat]

Version 2 onwards requires that the DB is saved in the KDBX4 format. If this causes compatibility problems for any users and the incompatible ports/software cannot be upgraded or replaced with a modern implementation, we would accept a pull request adding support for storing KPRPC configuration in an Advanced String (as per the historical approach in earlier KeePassRPC versions).

Minimum requirements:

• A plugin-level configuration option in the config file that forces this behaviour instead of the default KDBX4 behaviour
• Save config into a "KPRPC JSON V2" Advanced String (protected)
• Functionality to migrate between the V2 advanced string and Custom Data (to provide an upgrade path when a user is able to update to KDBX4)
• Documentation and warnings in relevant locations that using this feature will break KDBX compatibility with some software (e.g. Kee Vault)

Optional:

• UI in configuration dialog to change the setting

Feel free to split this into individual issues or PRs as you see fit and feel free to ask if you want any further guidance before getting started on the implementation.

[luckyrat]

Since KeePass 2.57 will save databases in the KDBX 4.0 or 4.1 format by default, the importance of this task has reduced further. If you need support for KeePassRPC 2 in versions of KeePass before 2.57 and the need to retain KeePassRPC data within KDBX 3 format, please leave a comment with more information about your situation and plan to implement the feature requirements specified above.

I'll re-open this issue if a need and plan are presented in the next year but otherwise we can treat this as a clean break with the old format.