v3.3.1: ZkSync Update

- fix rpc-definitions to handle nullable return-types
- add tx history to zksync 
- add tx details to zk_sync
- toNumber now handles null and undefines without converting it to 0
- add SIGN_EC_PREFIX as signer-type supporting EThereum Signing Messages
- add support for REST-APIs supporting get-requests without payload
- add sha256 util function
- bugfix converting bytes-arrays as return type for abidecode.
- support clear cache in js
- changed account-API in JS by supportting key management methods
- add getBlockcCount for btc
- added api-generator
- fix signer-delegation bug in JNI-Interface, which caused a segfault
- update zksync-version and support for validFrom and ValidUntil
- req_find_required adds additional (optional) argument to define the context as return value.
- refactor core-api as independend module