Design SDK (and DPP) API

[shumkov]

Based on previous investigations (#2281, #2368, and #2369), we need to go through all identified use cases, and decompose them into:

• methods that will be called
• data objects that will be used
• potential issues/problems to solve

We need to group methods into some logical groups (modules, crates, etc.),

Things to consider:

• learning curve for new developers