v1.2.0-alpha.18: Unit tests for Azure scenarios (#495)

* Test firewall rule handling is able to process through the service layer

* Additional tests for authentication and the resource wrapper code

* Positive test case for CreateFirewallRule

* Fixed copyright and usings

v1.2.0-alpha.17

fix the backup file path of a db file that's created in different pla…

v1.2.0-alpha.16: Improve performance of Create Firewall Rule code (#489)

* Multi-thread server lookup to increase perf for multiple subscriptions

* Use parallel execution for listsubscriptions
- Also refactored parallel execution code to be a bit more generic

v1.2.0-alpha.15: Increase parsing thread stack size to avoid stack overflow (#486)

* Increase parsing thread stack size to avoid stack overflow

* Fix extra space

* Remove extra spaces

v1.2.0-alpha.14

Fixing more restore connection issues (#488)

v1.2.0-alpha.13

Create Firewall Rule support with a simple Resource Provider implemen…

v1.2.0-alpha.12

closing the default connections that are opned just for validating th…

v1.2.0-alpha.11: Fixing the bug with connections on database make restore fail (#473)

* closing the connections that don't need to be open and keeping track of the connections that should stay open

v1.2.0-alpha.9

fixed indexing and selection logic (#471)

v1.2.0-alpha.10: Port Azure code from SSDT to the tools service (#477)

Porting of the vast majority of Azure-related code from SSDT. This is very large, so I want to put this out as one large "lift and shift" PR before I do the tools-service specific JSON-RPC service handlers, connect a new account handler (as the code to get necessary info from accounts and subscriptions isn't fully complete) and add tests over these

**What's in this PR**:

- Created 3 new projects:
  - Microsoft.SqlTools.ResourceProvider will host the executable that accepts requests for Azure-related actions over the JSON-RPC protocol. This must be separate from other DLLs since a direct dependency on the Azure SDK DLLs fails (they're NetStandard 1.4 and you can't reference them if you have RuntimeIdentifiers in your .csproj file)
  - Microsoft.SqlTools.ResourceProvider.Core is where all the main business logic is, including definitions and logic on how to navigate over resources and create firewall rules, etc.
  - Microsoft.SqlTools.ResourceProvider.DefaultImpl is the actual Azure implementation of the resource provider APIs. The reason for separating this is to support eventual integration back into other tools (since their Azure and Identity services will be different).
- Implemented the AzureResourceManager that connects to Azure via ARM APIs and handles creating firewall rule and querying databases. The dependent DLLs have had major breaking changes, so will need additional verification to ensure this works as expected
- Ported the unit tests for all code that was not a viewmodel. Viewmodel test code will be ported in a future update as we plumb through a service-equivalent to these. Also, the DependencyManager code which has overlap with our service provider code is commented out. Will work to uncomment in a future update as it has value to test some scenarios

**What's not in this PR**:
- Identity Services. We currently just have a stub for the interface, and even that will likely change a little
- anything JSON-RPC or registered service related. These will be adapted from the viewmodels and added in a separate PR