Proto/RPC: Improve RPCs consistency (`frozen_balance`)

Context

Improve consistency in RPCs by using plural nouns for frozen balance fields.

It could break some scripts and thus if merged should be mentioned in changelogs.

Fixes: #463 (closed)

Manually testing the MR

• Launch a sandbox node
• Bake one block
• Check the frozen balance for the account that baked the bloc using
  tezos-client rpc get /chains/main/blocks/head/context/raw/json/contracts/index/<PKH>/frozen_balance/0
  The answer should contain only plural nouns :)

Checklist

• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, the Development Version section of CHANGES.md for everything else).
• Select suitable reviewers using the Reviewers field below.