`Transaction::note` is a base64 string, but should probably be decoded.

[AlterionX]

Is your feature request related to a problem? Please describe.
The note for Transactions retrieved from the indexer is a base64 string. This is rather unexpected.

Describe the solution you'd like
The base64 string should be decoded 99% of the time. I'd say 100, but I'm sure there are use cases.

Describe alternatives you've considered
Leaving it as is, mostly.

Additional context
Possibly related to #98

[ivnsch]

We could use the same Transaction / ApiTransaction struct everywhere (modified to support deserialization from JSON and MessagePack). It would have to be checked that the indexer's Transaction is meant to be the same instance / has the same fields https://developer.algorand.org/docs/rest-apis/indexer/#transaction.

A variant could be to have a new version of ApiTransaction (which is meant to be 1:1 with the API model) that deserializes from JSON and convert it manually to Transaction (which is the domain representation).

On a more local level - if these are not possible, I agree that we should decode the note (and perhaps add convenience to convert it to e.g. utf-8 text).

Relates also to #142.