Replace Gson with Jackson in metrics-lib

Related to Onionoo ticket #25848 (moved), we should replace Gson with Jackson in metrics-lib.

Otherwise, if we don't switch here, Onionoo will have to include two dependencies in order to parse CollecTor's index.json using metrics-lib and to format and parse its various JSON files in its status/ and out/ directories.

Generally, we'll want to switch in other applications that depend on metrics-lib, and then we'll want to have a single dependency for JSON parsing/formatting in those applications.

Setting priority to high, because we should ideally put out a release with this change in the next few days and then releases of all other applications by the end of this month.