feat: prevent fetch on init

[kwasniew]

About the changes

Prevent fetching flags on init when storage has recent data (TTL not expired). This is a port of JS version Unleash/unleash-proxy-client-js#224

Important:

• I'm using base64 encoding instead of sha256 hashing as in the JS version. It doesn't require any crypto libraries and we still store the flags in storage as plain text

Important files

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