[FC-0099] docs: add 0004 Authorization Technology Selection ADR

[MaferMazu]

Description

This PR adds the ADR regarding the technology to be used for implementing the authorization system in Open edX.

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[bmtcril]

I think this is good to go once @mariajgrimaldi 's feedback has been addressed

[sarina]

Just a friendly reminder to please put FC numbers in the titles of PRs. Thank you!

[MaferMazu]

@rodmgwgu @bmtcril @mariajgrimaldi @sarina, sorry for the late addressing. This is ready for another cycle of reviews ✨

[mariajgrimaldi]

One last comment to address. The rest is already covered in the architecture, so we're good to go. Thanks for taking care of all my comments!