Developer dapp challenges

[dantaik]

I steal this idea from Marcus Wentz. Lets create a list of dapp challenges so developers can submit and earn a reward. The difficulty shall increase over time.

[dionysuzx]

i'm not sure if we can make this into alpha-3 but we'll try to and keep it triaged here.

also wanted to share this (possibly related), optimism quests: https://app.optimism.io/quests . maybe these dapp challenges are a form of a "quest"

[2manslkh]

Possibly move this to the grants repo? @dantaik @d1onys1us