Add UserAgentManager, for adding custom user agents outside skill context

[nikhilym]

Description

This commit introduces the following changes :

• A static user agent manager, that will handle component registration and retrieve user agent
• skill builder changes, to use UserAgentManager static class, for appending SDK user agent.

Motivation and Context

To allow frameworks built on the SDK, to provide user agent information without registering themselves to the skill builder. This way, frameworks can register user agents and create a skill context outside of skill builder, and still get the required user agent from the SDK.

Testing

Unit tests added.

Screenshots (if appropriate)

Types of changes

• [ ] 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

• [x] My code follows the code style of this project
• [ ] My change requires a change to the documentation
• [ ] I have updated the documentation accordingly
• [x] I have read the README document
• [x] I have added tests to cover my changes
• [x] All new and existing tests passed

License

• [x] I confirm that this pull request can be released under the Apache 2 license

[nikhilym]

Refactored the code around the nit comments. Since the functionality is the same , I am going ahead and merging the PR. Thanks for reviewing.