add salt to EIP712 domain

[alpa-coder]

What does this PR do?

Adds salt to the EIP712Domain. According to the EIP, a salt can be added to the domain:

an disambiguating salt for the protocol. This can be used as a domain separator of last resort

I double checked the results with what I got back from an EIP712 demo together with metamask. I also verified it in remix.

Where should the reviewer start?

4 changes:

• Added the salt in StructuredData - EIP712Domain
• Made sure the salt is not present when it is null (StructuredDataEncoder)
• Added a valid JSON test file with salt
• Created a new test to validate the result

Why is it needed?

If you use a valid EIP712 json with a salt, you get a deserialisation exception (unknown field).
Since this is part of the EIP it should be implemented.

[iikirilov-wmt]

LGTM - although someone needs to decide something about backwards compatibility something

[alpa-coder]

LGTM - although someone needs to decide something about backwards compatibility something

I checked, inside @JsonProperty the required is by default false:

boolean required() default false;

All tests inside "StructuredDataTest.java" still pass, which still has examples without salt and they are still green 👍
A test was added (and example json file) that tests the structured data with salt. I did not touch the ones without salt.

[conor10]

This pull request has been mentioned on Web3 Labs Community. There might be relevant details there:

https://community.web3labs.com/t/web3j-v4-8-0-released/112/1