Add setters to PrefixRecord and add methods to write back a PrefixRecord to JSON

[iamthebot]

Currently, in py-rattler a PrefixRecord is essentially read-only. However, to facilitate use cases where we want to manipulate a record (and optionally write it back) we'd also want setters.

[baszalmstra]

Id be happy to accept contributions. 😄

[iamthebot]

@baszalmstra looks like #663 is now in progress for this.

[jaimergp]

Also interested in this. Right now I have to dump to a temporary JSON just to instantiate new rattler.PrefixRecord objects from Python.

[iamthebot]

@Wackyator need help finalizing #663 ?

[baszalmstra]

I spoke with @Wackyator and I think its fine to take it over!