feat(core): add farcasterTimeToDate to farcaster/core

**What is the feature you would like to implement?**
add the farcasterTimeToDate function to farcaster/core

**Why is this feature important?**
to avoid this https://github.com/farcasterxyz/hub-monorepo/blob/main/packages/shuttle/src/example-app/app.ts#L37

**Will the protocol spec need to be updated??**
No