Standard JSON input / output types

[erak]

Part of #16394. Depends on #16586.

Description

In order to modularize the test execution framework, it was decided to settle on JSON as the common input / output data type for compilers under test. Since not only the testing framework, but also the CLI, which is planned to interface with the StandardCompiler in the future, would benefit from this common types. Furthermore, these common data types could be used where (de)serialization is currently duplicated (see e.g. StandardCompiler input parsing).

Design goals

• mirror schema of standard JSON input / output as closely as possible
• untangle schema and semantics (this layer only validates the input / output format, but does not add any semantics)
• have data types and (de)serialization in one place
• (de)serialize using nlohmann's conventions (via ADL for to_json/from_json)
• support lazy-deserialization

Open questions

Should StandardJSONOutput live inside test namespace for now, since the test execution framework is the only place where we need the outout right now?

TODO

• add unit tests
• clarify general error handling strategy (e.g. when to throw)

Input

• do not hard-code output selection

Output

• introduce Cached<T> for Contract members that should lazy-load
• add error severity

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