[200 SigmaUSD] Implement groupGenerator global var

[greenhat]

• Add enum variant GroupGenerator to ergotree-ir::mir::global_vars::GlobalVars;
• Write serialization roundtrip tests for all GlobalVars variants (PBT will be suffice, see serialization::expr::tests::ser_roundtrip);
• Add GlobalGenerator enum variant handling in ergotree-interpreter::eval::global_vars;

Reference Scala version:
IR node and eval - https://github.com/ScorexFoundation/sigmastate-interpreter/blob/b60edae20f2af3352e4064d139d45a15fc507f7d/sigmastate/src/main/scala/sigmastate/Values.scala#L691-L701

[greenhat]

Bounty is sent. Discussed and reduced to 150 Sigma USD due to some parts that were implemented earlier.