[#71] `Field name` and `Field contents` unifying

[DK318]

Description

Problem

We are using different naming for field name and field contents in core and CLI modules. However, in design doc we have these things unified.

Solution

Renamed fieldKey and fieldValue stuff to fieldName and fieldContents respectively.

Related issue(s)

• Fixes #71

Fixed #

:white_check_mark: Checklist for your Pull Request

Related changes (conditional)

• Tests

  • [ ] If I added new functionality, I added tests covering it.
  • [ ] If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.

• Documentation

  • [x] I checked whether I should update the docs and did so if necessary:
  • README
  • Haddock

Stylistic guide (mandatory)

• [x] My commits comply with the policy used in Serokell.
• [x] My code complies with the style guide.