supportsSubscriptions added to providers

[nivida]

This method will be used internally and can be used in the custom modules created with the Web3 Module API.

Description

Fixes #1015

Type of change

• Enhancement

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 warnings.
• I have updated or added types for all modules I've changed
• Any dependent changes have been merged and published in downstream modules.
• I ran npm run test in the root folder with success and extended the tests if necessary.
• I ran npm run build in the root folder and tested it in the browser and with node.
• I ran npm run dtslint in the root folder and tested that all my types are correct
• I have tested my code on an ethereum test network.

[coveralls]

Coverage decreased (-0.1%) to 95.29% when pulling 73f6878 on issue/1015 into 02f2866 on 1.0.

[coveralls]

Coverage increased (+0.002%) to 95.374% when pulling 6aa8ebb on issue/1015 into 9e3d016 on 1.0.