Amount.toString should support FormatSpec

[Geod24]

Currently our formatting for Amount is very basic and leads to some surprise.

agora/source/agora/common/Amount.d

Lines 79 to 92 in 4ae6db2

	/// Pretty-print this value
	public void toString (scope SinkT dg) const @safe
	{
	formattedWrite(dg, "%d", this.value);
	}
	/// Also support for Vibe.d serialization to JSON
	public string toString () const @safe
	{
	string ret;
	scope SinkT dg = (in v) { ret ~= v; };
	this.toString(dg);
	return ret;
	}

We should support %d to get the absolute value like currently, but being able to get 40,000 or 40,000.000,000,1 (up for discussion) would be nicer.

[Geod24]

Context:
https://github.com/bosagora/faucet/blob/834e16342aa5699ded55b90425734012e8135276/source/faucet/main.d#L535

Prints:

Feb 11 05:57:13 eu-001.bosagora.io docker[73427]: 2022-02-11 05:57:13,329 Info [faucet.main] - Sending 400000000000 BOA to boa1xz3qed5979y0v0mjjsndh5lcjmyvp9hrcsu78qs4mjw8gugqnkxx25e3qfd

Which is wrong, it's sending 400000000000 (400,000,000,000) units, which is 40k BOA.