[notification hubs] Adding Xiaomi support

[mpodwysocki]

Packages impacted by this PR

• @azure/notification-hubs

Issues associated with this PR

Describe the problem that is addressed by this PR

Adds Xiaomi support for Track 2 SDKs with registration, installation, send and send outcome operations.

What are the possible designs available to address the problem? If there are more than one possible design, why was the one in this PR chosen?

Are there test cases added in this PR? (If not, why?)

yes

Provide a list of related PRs (if any)

• Add xiaomi notification azure-notificationhubs-dotnet#261
• Add Xiaomi Registration classes and Xiaomi template registration classes azure-notificationhubs-dotnet#260
• Add Xiaomi Installation azure-notificationhubs-dotnet#259

Command used to generate this PR:**(Applicable only to SDK release request PRs)

Checklists

• Added impacted package name to the issue description
• Does this PR needs any fixes in the SDK Generator?** (If so, create an Issue in the Autorest/typescript repository and link it here)
• Added a changelog (if necessary)

[azure-sdk]

API change check

APIView has identified API level changes in this PR and created following API reviews.

azure-notification-hubs

[jeremymeng]

LGTM except the version

[mpodwysocki]

/check-enforcer evaluate

[mpodwysocki]

/check-enforcer override