Add command stubs for the following commands:

• donorStartMigration
• donorWaitForMigrationToCommit
• donorForgetMigration

These commands should be implemented using TypeCommand and should go in src/mongo/db/commands/migrate_tenant_cmds.cpp and src/mongo/db/commands/migrate_tenant_cmds.idl. The scram auth option and x.509 auth option fields for the donorStartMigration command can be omitted for now. 

Refer to txn_two_phase_commit_cmds.cpp and txn_two_phase_commit_cmds.idl for examples.