Client Signing [enhancement] v2

[malamalca]

I am revisiting client signing as of #35 . I've implemented complete client signing process using SAPP with minimal changes to original code.

The steps taken from #35 :

• synced with main branch https://github.com/malamalca/sapp
• implemented complete process of signing on client, using custom webservice from https://github.com/malamalca/arhint-signer (not fixed to this library, any other client system would do - eg hwcrypto.js )
• tried (AI did that) to describe the process in https://github.com/malamalca/sapp/blob/main/CLIENT_SIGNING_README.md
• there are some new functions in a newly created https://github.com/malamalca/sapp/blob/main/src/helpers/ClientSideSigning.php
• updated CMS.php to share funcs with client signing (some of the inline code moved to separate functions)

Could you look at the implementation and state your opinion, if this functionality belongs to your project?