Replace usage of Klaxon library with Jackson

[aldoborrero]

Currently, we are using a mixture of jackson libraries for several parts of the project that can be found on:

• buildSrc: deserialization from YAML | auto code generation of web3 entities and so on
• base: allows to specify a custom ClassLoader

And klaxon that can be found on testutil package.

In order to simplify things, and dependencies, I propose to migrate testutil as well to use directly jackson instead.