Use signature to generate point for PRE encryption

[peterlimg]

Changes

• Use signature to generate point for PRE encryption. The client.Sign() will use primary key or call zauth to sign the tag accordingly to get the signature. Both cases will return the same signature for the wallet with the same client id.

Fixes

Tests

Tasks to complete before merging PR:

• Ensure system tests are passing. If not Run them manually to check for any regressions 📋
• Do any new system tests need added to test this change? do any existing system tests need updated? If so create a PR at 0chain/system_test
• Merge your system tests PR to master AFTER merging this PR

Associated PRs (Link as appropriate):

• blobber:
• 0chain:
• system_test:
• zboxcli:
• zwalletcli:
• Other: ...