Add types for subscription.on(connected)

[cgewecke]

Description

Adds type definition for the subscription 'connected' event.

Don't see a unit test verifying that the value passed to the connected callback is a string but I checked this manually and looks right.

Fixes #3319

Type of change

• Bug fix (non-breaking change which fixes an issue)

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 85.445% when pulling d9ac8c1 on issue/3319 into 7386be2 on 1.x.

[nivida]

LGTM! Test cases for the connected listener do exist in the PRI branch and should get merged soon ™.