Introduce App Attest

[lihter]

Thank you for submitting a Pull Request to the OWASP MASTG. Please make sure that:

• [✅] Your contribution is written in the 2nd person (e.g. you)
• [✅] Your contribution is written in an active present form for as much as possible.
• [✅] You have made sure that the reference section is up to date (e.g. please add sources you have used, make sure that the references to MITRE/MASVS/etc. are up to date)
• [✅] Your contribution has proper formatted markdown and/or code
• [✅] Any references to website have been formatted as [TEXT](URL “NAME”)
• [✅] You verified/tested the effectiveness of your contribution (e.g.: is the code really an effective remediation? Please verify it works!)

If your PR is related to an issue. Please end your PR test with the following line:
This PR closes #1913 .

Preview of the changes:

[cpholguera]

Thanks a lot @lihter!