Generalize support for external providers

[glethuillier]

The tool only handles two specific external providers: a default (free) one, and a custom (paid) one.

The implementation could be improved by generalizing the support for external providers. Notably, a more robust way of transforming the raw responses into raw transaction and transaction models could be suggested.

Here are some possible checks:

• Ensure that the API key is valid (i.e. perform one request and check that there is no authorization issue before running the full scan)
• Validate the contents of the response: is it the expected structure? Are the types valid?

Refactor the code to have some kind of dispatcher to select the relevant provider.

Note that the requests underlying implementation should be improved beforehand.

In this context, see also: #7 (review)