refactor: Switch to packbackbooks/lti-1-3-php-library

[cgb37]

Due to the possible abandonment of the original lti-1-3-php-library and need to address vulnerability issues with phpseclib version 2, SP is going to switch to a forked version.

Here is the issue:
1EdTech/lti-1-3-php-library#75

and Dependabot:
phpseclib does not properly limit the ASN1 OID length #10
https://github.com/subjectsplus/SubjectsPlus/security/dependabot/10
An issue was discovered in phpseclib 1.x before 1.0.23, 2.x before 2.0.47, and 3.x before 3.0.36. When processing the ASN.1 object identifier of a certificate, a sub identifier may be provided that leads to a denial of service (CPU consumption for decodeOID).