Improving currencyrate plugin

[enaples]

Important

26.04 FREEZE March 11th: Non-bugfix PRs not ready by this date will wait for 26.06.

RC1 is scheduled on March 23rd

The final release is scheduled for April 15th.

Checklist

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• The changelog has been updated in the relevant commit(s) according to the guidelines.
• Tests have been added or modified to reflect the changes.
• Documentation has been reviewed and updated as needed.
• Related issues have been listed and linked, including any that this PR closes.
• Important All PRs must consider how to reverse any persistent changes for tools/lightning-downgrade

Currencyrate plugin changes

As mentioned in #9010,

1. Check on RPC argument number added
2. Show max three decimal digits
3. Added source argument to the currencyrate RPC call
4. Added "Example" on RPC documentation

Info Needed

I would personally suggest to add a README.md file in the currencyrate plugin folder reporting how to configure new sources in the config file, plus some best practice using reckless to add and disable sources without restarting the node.

[Lagrang3]

Instead of moving from f64 to a string and then back again to f64, it would be better to do some purely mathematical operation.
eg. output = (input*1000.0).round() / 1000.0;

[daywalker90]

to always get 3 decimal places you must return a String:

fn round_to_3dp(price: f64) -> String {
    format!("{:.3}", (price * 1000.0).round() / 1000.0)
}

[enaples]

This method is not that reliable. See here the comment for very large number. That's why I opted for the f64 --> String --> f64 solution.

I need to return again f64 since the price is used to compute the median, so it is better to return f64 again in my opinion.

[daywalker90]

Oh, the linked issue said to always have 3 decimal places not at most 3. So this is fine then if we can live with always rounding toward 0.