Add bcmath_compat polyfill for servers without BCmath / GMP support

[nikosdion]

Pull Request for Issue #38485.

Summary of Changes

Added the BCmath_compat polyfill from the well–known and well–trusted phpSecLib. This library adds a userland (pure PHP) implementation of the BCmath functions used by the third party dependencies of the WebAuthn code for BOTH logging in AND Multi-factor Authentication. If you try to use WebAuthn on a server which has neither the BCmath nor the GMP PHP extensions enabled the polyfill will kick in and let WebAuthn work properly.

Testing Instructions

• Apply this PR on a Joomla 4.2-dev working copy.
• Run composer install to update the Composer dependencies.
• Run the resulting Joomla 4.2 on a server which has neither BCmath nor GMP installed.
• Set up MFA with WebAuthn for your user account.
• Log out and log back in.
• Proceed with the WebAuthn authentication.

Actual result BEFORE applying this Pull Request

You receive the error “Requires GMP or bcmath extension”.

Expected result AFTER applying this Pull Request

Everything works fine.

Documentation Changes Required

In the requirements section for Joomla 4.2 we should add something along the lines of:

• We recommend enabling one of the PHP extensions BCmath or GMP (they are not required but do increase the performance of the WebAuthn features).

[nikosdion]

There's one build failure in Drone but it seems it's not coming from this PR.

[richard67]

There's one build failure in Drone but it seems it's not coming from this PR.

I've restarted it, and this time it worked. Sometimes we have that problem.

[HLeithner]

Works for me without bcmath and gmp, funny note I also have to remove libsodium.

[roland-d]

Thanks everybody