Clemensgg/fetch all chains

[clemensgg]

This PR introduces the following changes:

• uses github api
• downloads full repo as zip
• periodic caching
• routes the /fetch calls to cache, if no cached data is found it fetches live
• introduced GET /mainets & /testnets (calls cache)
• number of workers can be set with env var NUM_WORKERS=x (default: 10)

[clemensgg]

rebased with main