rpc: Accept strings in AmountFromValue

Accept strings containing decimal values, in addition to bare values.

Useful from JSON-RPC implementations where it's not possible to have
direct control over the text of numbers (e.g. where numbers are always
doubles), and it's still desired to send an exact value.

This would allow users to post JSON content with numbers encoded like
{"value": "0.00000001"} instead of {"value": 0.00000001} which some
php/python encoders wrap into 1e-8, or worse.