Fix unity.data.br caching

[eordano]

Every time the explorer is loaded, at least on web, 44MB corresponding to the unity.data.br file are getting downloaded. Is this necessary? As it is part of the built files, we should be caching this to save bandwidth and time for the users.

Acceptance criteria: this data is now cached and loading is faster next time

[eordano]

We need to ensure unity build is always less than 42mb

[2fd]

Browsers have a limit on the size of the files that can cache (Chromium: 42Mb, Firefox: 52Mb). The low-hanging fruit would be splitting the unity build on files lower than 42Mb, if not possible, we would need a service worker, but that requires managing the expiration of our own cache.

Research Walkthrough: https://www.notion.so/decentraland/Cache-unity-data-br-on-explore-4382b0cb78184973af415943f708cba1
PoC for Browser Cache: https://github.com/2fd/poc-browser-cache