Add the ability to switch between K1 and K2 analysis for testing

[IgnatBeresnev]

The refactoring introduced in #3034 allows Dokka to use either K1 or K2 compiler for analysing Kotlin projects.

The ability to switch between K1 and K2 should be added, so that we can start regression testing on a wider range of projects.

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Blocked by: #3094

[IgnatBeresnev]

Discussion summary:

• The switch will be used for testing purposes only; A user-facing switch will be added separately and once K2 is stable enough for people to try.
• A single Gradle property is preferred (if possible), or some other way to dynamically change the analysis implementation without having to re-build Dokka.
• The switch should be toggleable in tests (i.e we should be run unit and integration tests in CI with K1/K2);
  • Setting up unit and integration tests to use the switch falls outside of the scope of this issue.
• The switch should indicate that it's an internal tool/API: internal in the name/path, or maybe even a warning printed to the logs.

Additional notes:

• Projects that are using K2-only features might fail when K1 is enabled.
• If K2 analysis is enabled, and the user project is still using K1, it should not interfere or cause any problems. Same principle applies vice versa.

[IgnatBeresnev]

Implemented in #3152