init_transaction module not extensible to custom transactions

[mitsuaki-u]

Expected behavior

logic/init_transaction module should be extensible to accommodate for new custom transaction types created by sdk user.

Actual behavior

logic/init_transaction module directly maps known transaction types to raw db format.

See issue #2424

Which version(s) does this affect? (Environment, OS, etc...)

1.7

[2snEM6]

Also elements/lisk-transactions/src/utils/validation/schema.ts:111 is setting the maximum value for BaseTransaction type to be 7. This constraint needs to be removed so we can register custom transactions with a type > 7