feat: support full user targeting on workflow USER TASK

[engalar]

Fixes #169

Summary

• Parser now reads both legacy UserSource and current UserTargeting BSON fields, recognizing all five $Type variants (MicroflowUserTargeting, XPathUserTargeting, MicroflowGroupTargeting, XPathGroupTargeting, NoUserTargeting)
• Added MicroflowGroupSource and XPathGroupSource SDK types with full BSON serialization/deserialization
• MDL grammar extended with optional USERS/GROUPS modifier: TARGETING [USERS|GROUPS] MICROFLOW/XPATH
• DESCRIBE output now correctly displays all targeting types including group targeting

Syntax

-- User targeting (existing, backward compatible)
TARGETING MICROFLOW Module.GetUsers
TARGETING XPATH '[System.UserRoles = ''[%UserRole_Manager%]'']'

-- Explicit user targeting (new)
TARGETING USERS MICROFLOW Module.GetUsers
TARGETING USERS XPATH '[...]'

-- Group targeting (new)
TARGETING GROUPS MICROFLOW Module.GetGroups
TARGETING GROUPS XPATH '[GroupType = ''Reviewers'']'

Test plan

• 6 new parser tests covering all targeting types (user microflow/xpath, group microflow/xpath, no targeting, legacy UserSource)
• mxcli check validates all syntax variants
• Full test suite passes
• Build succeeds