feat: support of multiple vshard groups

metadata argument from dql/dml_on_... methods from vshard.rs replaced by explicit timeout value.

trait Router extended with two methods: get_current_vshard_group_name, get_vshard_object_by_group_name.

Relates to picodata#713 (closed)

MR in picodata - picodata!1138 (merged)

Closes #493 (closed)

Ensure that

• New code is covered by unit and integration tests.
• Public modules are documented (check the rendered version with cargo doc --open).
• (if PEST grammar is changed) EBNF grammar reflects these changes (check the result with railroad diagram generator.

Next steps

• Cherry-pick to: none
• Update sbroad submodule in picodata/picodata.