Add method to sign generic transactions so they can be sent independently of plugin

[cypherhat]

Description

sign-tx method allows signing of raw transactions.

Motivation and Context

Several requests for this method had previously fallen on deaf ears. Cleaned ears. Implemented method.

How Has This Been Tested?

Not extensively at all.

Screenshots (if appropriate):

Types of changes

• [ ] Documentation enhancement
• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [ ] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [ ] I have read the CONTRIBUTING document.