Support parse_float (or some other name) in msgspec.json.decode

[jcrist]

Description

The standard library's json.loads function includes a parse_float hook. If configured, this will be called for any floating point number with the raw input str. This may be used to parse all float values as decimal.Decimal types by passing parse_float=decimal.Decimal.

We might want to support an option like this. Due to msgspec's design, we'd probably go with:

• Calling the option float_hook to mirror the other callback names
• If configured this would be called to decode any float value that didn't have an explicit type configured (e.g. type=Any only). This would effectively let the user configure how untyped float values are decoded - float values with specified output types wouldn't use this mechanism.

To parse all untyped floats as decimals you'd do something like msgspec.json.decode(msg, float_hook=decimal.Decimal).