Callback type for Accounts.signTransaction fixed

[nivida]

Description

This PR adds the callback parameters with his types to the type definition of the Accounts.signTransaction method.

Fixes #3278

Type of change

• Types

Checklist:

• I have selected the correct base branch.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• Any dependent changes have been merged and published in downstream modules.
• I ran npm run dtslint with success and extended the tests and types if necessary.
• I ran npm run test:unit with success and extended the tests if necessary.
• I ran npm run build-all and tested the resulting file/'s from dist folder in a browser.
• I have updated the CHANGELOG.md file in the root folder.
• I have tested my code on the live network.

[coveralls]

Coverage remained the same at 84.802% when pulling f1448b2 on issue/3278 into 4d57461 on 1.x.