SignalR - Add more properties to the models, add usage API and complete tests

[allxiao]

Description

The change is based on the spec PR: Azure/azure-rest-api-specs#3686 . We plan to release the version 0.10.0-preview for the Ignite.

Changes:

• Add version to the SignalRInstance model
• Add connection strings to the SignalRKeys model
• Add usage operations for quota limit related query
• Add tests

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[dsgouda]

@ArieShout will open a new PR once the stable swagger is checked in, Will reopen this PR if anything changes

[allxiao]

Hi @dsgouda , we plan to stay in preview before Ignite and work on stable api-version and SDK release after that.
Could you please help to reopen the PR? I have prepared the changes according to your review comments locally and will update after reopening.
Thanks,

[dsgouda]

Thanks for confirming. Reopening PR. Will review once changes are pushed

[allxiao]

Updated the code according to the review comments and rebased to the latest psSdkJson6 branch.

[dsgouda]

LGTM