## Issue being fixed or feature implemented
## What was done?
<!--- Describe your changes in detail -->
- `bls generate` and `bls fromsecret` rpcs will return `scheme` used to
serialise the public key. Valid returned values are `legacy` and`basic`.
- `bls generate` and `bls fromsecret`rpcs accept an incoming optional
boolean argument `legacy` that enforces the use of legacy BLS scheme for
the serialisation of the reply even if v19 is active.
## How Has This Been Tested?
## Breaking Changes
## Checklist:
- [x] I have performed a self-review of my own code
- [ ] I have commented my code, particularly in hard-to-understand areas
- [ ] I have added or updated relevant unit/integration/functional/e2e
tests
- [x] I have made corresponding changes to the documentation
**For repository code-owners and collaborators only**
- [x] I have assigned this pull request to a milestone
