Implement custom Json Marshaller for AssessmentReport struct

[sanyamsinghal]

Describe the changes in this pull request

• using this we can achieve different content in json output for assessment report vs ybd payload

Describe if there are any user-facing changes

NA

How was this pull request tested?

Existing test enough.

Does your PR have changes that can cause upgrade issues?

Component	Breaking changes?
MetaDB	No
Name registry json	No
Data File Descriptor Json	No
Export Snapshot Status Json	No
Import Data State	No
Export Status Json	No
Data .sql files of tables	No
Export and import data queue	No
Schema Dump	No
AssessmentDB	No
Sizing DB	No
Migration Assessment Report Json	No (not json actually, but struct has changed which shouldn't impact upgradability)
Callhome Json	No
YugabyteD Tables	No
TargetDB Metadata Tables	No