fix!: Allow ConfigCat provider to be used in server applications

[adams85]

This PR

The conficat-js package that the ConfigCat provider is based on is for browser applications only as it uses XMLHttpRequest under the hood. However, the provider might be used in server applications (like it was attempted in a Node.js application here, at around 2:00:00).

For multiplatform scenarios, ConfigCat provides the conficat-js-ssr package (that uses Axios under the hood). This provider should use that instead of conficat-js to provide a more extensive support for various JS platforms.

This PR also fixes another serious shortcoming of the provider: currently it cannot evaluate feature flags without setting targetingKey in the evaluation context. Simple feature flags may not use targeting at all, so requiring a targetingKey to evaluate them is obviously not correct.

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[lukas-reining]

Hey @adams85 thank you this change looks good!
Maybe we should also consider developing a client-sdk provider, as this is a server sdk provider. Which makes you change even more important.
We can release this, but it has to be a breaking change as the API changed.

[lukas-reining]

Looks good to me and thanks for the changes!

[adams85]

Thanks for the release, @lukas-reining. I did a quick test and found that the new version (v0.6.0) works fine.

Maybe we should also consider developing a client-sdk provider, as this is a server sdk provider. Which makes you change even more important.

Since our SSR SDK can handle both targets, I guess the client provider would be more or less just a duplicate of the server one (at least, in our case). I'd avoid maintaining another package if possible.

BTW, NPM packages kind of support multitargeting via the browser field. AFAIK, most of the bundlers support it nowadays. I have plans to look into the viability of this approach because it would be a godsend to us if we could merge our various JS SDK packages (or at least some) into one.