asset/btc/electrum: make floatString work for floats and strings

[ukane-philemon]

This PR improves the UnmarshalJSON method of type floatString.

[JoeGruffins]

Could you add a test that confirms it does what it should? I don't know exactly what we want to parse, like "1.31" and 0?

[ukane-philemon]

like "1.31" and 0

Yes, floatString should do both, master code parses only the former. This PR enables parsing both.

[buck54321]

Please provide some reasoning for why this change is needed.

[ukane-philemon]

Please provide some reasoning for why this change is needed.

I intend to use this in #2769, as a shared code since we deal with floatString there too, suggesting this as a smaller change to make floatString compatible with cases where the same fields could be represented as floats and strings depending on the asset. In other cases where we'd need this, we would simply add a type.