add structured logging feature

[fmeriaux]

Close #1932

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• Check if the project compiles by running sbt compile
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[fmeriaux]

@adamw I've tried something, but I'm not sure if I can add a unit test on this feature?

[adamw]

Sorry not sure how I missed this. Testing logging is difficult, but if you have an idea on how to approach this, go ahead :)

[fmeriaux]

We should be able to test DefaultLog by making a SpyLogger that buffers what we send it.

I'm also going to separate the context construction into a separate class to test it in isolation and allow users to provide their own implementation of the context.

[fmeriaux]

@adamw Can you give me a clue as to how to solve the CI failure? I'm having trouble understanding what's wrong.

[adamw]

It seems that once again the JS builds are broken because of problems with Chrome version / Selenium / some plugin. I'll take a look once I get some time :)

[adamw]

This looks great, thanks for your work! I fixed the JS build & left two minor comments on the code

[adamw]

Thanks for the fixes :)