Support exchange api key in header (example: apilayer.com)

[develCuy]

Make sure these boxes are checked! 📦✅

• You have a nightly compatible version of rustfmt installed

rustup component add rustfmt --toolchain nightly

• You ran cargo +nightly fmt on the code base before submitting

Reference: https://apilayer.com/marketplace/exchangerates_data-api (Tap: Documentation)

When trying to register to https://exchangeratesapi.io/ user is prompted to register at apilayer.com instead. The access_key was required in the URL but the new provider requires to pass it as apikey and as a header.
Two new variables were added to .env.example as example of the new logic. The proposed change is backwards compatible.

[github-actions]

CLA Assistant Lite bot All contributors have signed the CLA ✍️ ✅

[fernando-syslabs]

I have read the CLA Document and I hereby sign the CLA

[develCuy]

I have read the CLA Document and I hereby sign the CLA

[fmrsabino]

Hey @develCuy ! Thank you for your contribution and apologies for only providing an update to this PR now.

The team was quite busy in rewriting a new version for this service which can now be found under https://github.com/safe-global/safe-client-gateway-nest.

We do not expect to provide further support on the Rust implementation right now but feel free to fork this project and run the instance with your own changes. Additionally if you feel like this should be implemented on the the implementation I'd open a new issue there for discussion.

Thank you!