[Storage] Connection string keys should be case-insensitive

[joheredi]

Currently, the Storage SDKs parse connection strings assuming Keys are case-sensitive. However, keys are case-insensitive according to ADO.NET docs: https://docs.microsoft.com/en-us/dotnet/framework/data/adonet/connection-strings#connection-string-syntax

We should update our logic to handle keys as case-insensitive

[ljian3377]

Nice catch. I see a .NET PR addressing the same issue: https://github.com/Azure/azure-sdk-for-net/pull/17642

We also don't deal with secondary endpoint for connect string. https://github.com/Azure/azure-sdk-for-net/blob/master/sdk/storage/Azure.Storage.Common/src/Shared/StorageConnectionString.cs