[Feature Contribution]: support Electricity Maps Free as a new data source

[FabioTurati-NTT]

What happened?

Electricity Maps provides three different services:

• The paid service
• The free trial of the paid service, which lasts one month
• The free version, which is also called CO2 Signal

This feature adds support to the third service, the free one.

Related ADR

There's an ADR about how to treat this new data source, which hasn't been approved yet. However, the implementation can be started without waiting for that, because this feature is about what happens "under the hood", and the ADR is about whether and how this should be exposed to the user. So even if the ADR is not approved, this will not result in wasted effort, because this part will be required in any case.

Service name

There has been some discussion about its name, whether to call it "Electricity Maps Free" or "CO2 Signal". A member of Electricity Maps, @kathrinsiller, commented that

One option we propose would be to move forward with the integration of our free tier hosted on CO2 signal (but keeping the Electricity Maps Free name)

So the name should be "Electricity Maps Free".

Previous work

• This feature had already been implemented with branch feat/electricity-map (merged with PR 75). Unfortunately the code base has changed in the meanwhile, and that branch does not compile anymore, so it must be adapted
• This feature has also been implemented by Microsoft (issue, branch). However we had a discussion with @JenMadiedo, @pritipath and we agreed that it would benefit the project to have contributions from other members. Moreover, this is a great learning opportunity for me. We agreed that I will work on this, and they will support me if I need help, and/or review my PR when it's ready.
• I also have an initial implementation

Code of Conduct

• I agree to follow this project's Code of Conduct

Feature Commitment

• I commit to contributing this feature as a PR and working with the GSF to merge this feature into the Carbon Aware SDK.