Create 03_API_Broken_Function_Level_Authorization.md

[irgoncalves]

Add API BFLA Testing

This PR is part of issue #5.

What did this PR accomplish?

Add Broken Function Level Authorization - BFLA testing to the Testing Guide

[github-actions]

The following issues were identified:
document/4-Web_Application_Security_Testing/12-API_Testing/03_API_Broken_Function_Level_Authorization.md:41 MD046/code-block-style Code block style [Expected: fenced; Actual: indented]
document/4-Web_Application_Security_Testing/12-API_Testing/03_API_Broken_Function_Level_Authorization.md:69 MD046/code-block-style Code block style [Expected: fenced; Actual: indented]

[github-actions]

The following issues were identified:
document/4-Web_Application_Security_Testing/12-API_Testing/03_API_Broken_Function_Level_Authorization.md:41 MD040/fenced-code-language Fenced code blocks should have a language specified [Context: "triple-backtick"]

[kingthorin]

I'm not going to be able to review this for a few weeks due to work and just life stuff.

Please know it's appreciated and I'm not ignoring it.