Introduce CompileTarget and plumb it through the Compiler API

[lmondada]

I've reworked #4569 to work better with @Renaud-K's proposed changes in #4557.

This PR sets up the plumbing to define compilation options in a new CompileTarget struct. This struct is currently constructed and populated by the QPU class using the QPU::getCompileTarget method. The method takes an executionContext as argument. It is currently unused but is paving the way to remove any dependency on the execution context within Compiler::runPassPipeline (and hence also from Compiler::lowerQuakeCode): the compiler pipeline would instead be fully determined from the CompileTarget.

In the future, we may choose to separate CompileTarget out of QPU and have the source of truth be outside of the QPU class.

[github-actions]

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