Proposed changes

There are several services that use custom capabilities , e.g. wdio:electronServiceOptions to allow to configure the service behavior for a specific capability. It was currently not possible to extend the capability object. This PR introduces a new WebdriverIO.Capabilities interface that allows to extend the capabilities.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Checklist

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• I have added proper type definitions for new commands (if appropriate)

Further comments

n/a

Reviewers: @webdriverio/project-committers