Start RetroAchievements integration

[ChaseParate]

I've just set up the core infrastructure and imported the rcheevos library.

The feature is set behind the ENABLE_RETROACHIEVEMENTS CMake flag, and I put a spot in the configuration menu to authenticate with RA:

I figured I should submit this PR now in case there are potentially any major infrastructural changes/refactors requested. I'm pretty new to CMake (and this codebase as a whole), so all feedback is welcome!

[github-actions]

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[ChaseParate]

My username is ChaseParate and the logo is from Azahar.

[github-actions]

Verification successful! Pull request has been reopened. Please also edit your PR description to remove the block of text between --- to make the description easier to read.

[ChaseParate]

I realized I wasn't actually using the user-agent header... Just fixed and cleaned up some surrounding code.

[ChaseParate]

Fixed merge conflict from libretro PR.

[PabloMK7]

Thanks! I will convert this to a draft anyways to not give the impression it's something imminent, as a lot of things need to be figured out still.

[ChaseParate]

Sounds good. I'm a bit swamped from school/work at the moment, but I will try keeping up to date on this, and I will have significantly more time to focus on this over the summer.