Re-implement prompt builder & make it public

[Vladislav0Art]

Description of changes made

1. Refactor the implementation of PromptBuilder and introduce validation of the prompt keywords inserted.
2. Cover PromptBuilder with unit tests.
3. Make PromptBuilder public so client code has more control over prompt generation (required by the Fleet plugin).

Why is a merge request needed

1. Introduce unit tests for functionality from the core module.
2. Implement PromptBuilder in a more robust manner.

Other notes

Closes #291

What is missing?

• I need to publish a new core library version. The version should be 2.0.8 because previously PromptBuilder was not accessible (i.e., marked as internal). Therefore, no client code can observe the introduced modifications to PromptBuilder's API. 3.0.0 because API of PromptKeyword has changed (namely, text member field was removed).
• Implement the test cases for the remaining methods.
• Merge this branch only after this PR is merged, as the latter has a core module version changed to 2.0.5, which should come into development before the current version 2.0.7.
• I have checked that I am merging into the correct branch

[Vladislav0Art]

Published 3.0.0 version of testspark-core.