[SDK] Ensure all extrinsics in prediction markets pallet have SDK interfaces

[lsaether]

Please catalogue and ensure all the extrinsics in the Prediction Markets pallet has the appropriate SDK functions.

[morkeltry]

[O] admin_destroy_market - not implemented - don't implement admin functions for now.
[O] admin_move_market_to_closed - not implemented - don't implement admin functions for now.
[O] admin_move_market_to_resolved - not implemented - don't implement admin functions for now.
[X] approve_market
[X] buy_complete_set - exists, tested
[X] cancel_pending_market
[X] create_categorical_market
[X] create_scalar_market - createNewMarket only creates categorical markets - make an equivalent for scalar markets before multifunc
[X] deploy_swap_pool_for_market - exists, may need further testing due to #69
[X] dispute
[O] global_dispute - don't implement
[X] redeem_shares - exists, needs further testing due to #69
[X] reject_market
[X] report
[X] sell_complete_set - exists, needs further testing due to #69
[O] outcome_assets - internal function - don't implement