Use constructor in TargetSpec, allow union in builder

[javiermtorres]

A normal constructor can now be used with TargetSpec. Also, the builder config dict target accepts either a str or a prebuilt Target Spec. The stubtest uses the right _core module now.

Since the TargetSpec is now part of the EncoderfileConfig, it needs to derive JsonSchema. Let me know if that would be an issue. From the top of my head I don't know if we currently test schema generation and check something on it.

The version param takes None as default to make it consistent with the Rust-side option. However, it might be more appropriate to signal '0.1.0' as default, as it is the real value that will be set up when the default is used. A discussion for another day 😅

[codecov-commenter]

Codecov Report

❌ Patch coverage is 7.14286% with 13 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
encoderfile-py/src/builder.rs	0.00%	13 Missing ⚠️

Files with missing lines	Coverage Δ
encoderfile/src/builder/base_binary/target_spec.rs	39.77% <ø> (ø)
encoderfile/src/builder/config.rs	76.86% <100.00%> (-0.27%)	⬇️
encoderfile-py/src/builder.rs	0.00% <0.00%> (ø)

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