blockchain2john.py handling of old Base64 wallets is prone to user error

See #4872

$ ../run/blockchain2john.py -h
usage: ../run/blockchain2john.py [blockchain wallet files]

optional arguments:
  -h, --help  show this help message and exit
  --json      is the wallet using v2 format?
  --base64    does the wallet contain only a base64 string?

The problem with the --base64 option (for very old version wallets) is that if you don't give it where needed, you still get output that looks like a perfectly fine hash, but is uncrackable.

We don't seem to have any blockchain wallets in https://github.com/openwall/john-samples, maybe a first step is adding some (hopefully from https://github.com/gurnec/btcrecover/tree/master/btcrecover/test/test-wallets).

Then, see if we can implement a trivial check for whether a file is 100% Base-64 (while also not a too small subset of it) and either automagically infer the --base64 option, or at least print a warning.

While at it, we could possibly implement logic for auto-detecting json as well.

Start with run/blockchain2john.py and the behavior described in #4872, then review available wallet samples from john-samples and the referenced btcrecover test-wallets. Determine how old Base64 wallets and JSON wallets can be identified, and make completion observable through correct handling or a clear warning when --base64 is omitted.