refactor: proper support for composite commands such as 'bls generate'

[knst]

Issue being fixed or feature implemented

We have composite commands such as 'bls generate' that do not exist in bitcoin's implementation of rpc. It doesn't let to backport yet bitcoin#18531 which enforced extra checks for arguments name (name of rpc and list arguments in rpc help and actual implementation must match).

What was done?

This PR improves support of composite commands in Dash Core. New style of composite commands are applied for bls composite commands: bls generate and bls fromsecret as proof of concept. Once this PR is merged, I will provide similar fixes for other "compose" rpc commands: protx, masternode (and everything else if any).

Beside better validation of arguments and command names, it improves suggest menu in Qt app (see a screenshot) for composite commands.

How Has This Been Tested?

Run unit and functional tests. Also extra tests to conduct in qt app:

• check suggest for 'bls ....' and 'help bls ...'
• check output of next commands:

  help bls
  help bls generate
  help bls from secret
  bls
  bls generate
  bls generate 1

• also let's see that old-fashion composite commands are not broken:

  help protx
  help protx diff
  protx diff 00000021c7604b9992254f9f1ed91de5d65eaade33c773abea63a7b0e93293ee 000000e44f9894838ebf768b464177cfce8859dcf92b0509f5c2fba774315996

Breaking Changes

N/A

Checklist:

• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have added or updated relevant unit/integration/functional/e2e tests
• [x] I have made corresponding changes to the documentation
• [x] I have assigned this pull request to a milestone

[knst]

just note for history: it seems as this PR breaks help for composite commands that has not been refactored yet, such as help protx list