Add historical exchange rate data to API

[PodTheCoder]

Problem:
Currently, there is no easy way to fetch historical exchange rate data from the official index utils API.

Rationale:
There are several potential use cases. I'd to highlight this use case one: A node owner might need historical exchange rate data of node earnings for tax purposes. Governments still want earnings from crypto to be paid in FIAT so it's good to allow compatible interaction.

Solution:
The index-utils already supports fetching the current exchange rate via exchangeRate.current():
https://github.com/edge/index-utils#exchange-rate

The SDK could be extended with exchangeRate.historical(date) or a similar function name.

[PodTheCoder]

@annybs Related to the Discord suggestion: https://discord.com/channels/371989135172567051/814860932723245156/1129018624296886312