azurerm_storage_account - exposing a formatted Connection String for Blob access

[tombuildsstuff]

Chatting with @radeksimko during #130 - I realised we were having to manually construct the Connection Strings for accessing Storage Accounts. This PR exposes a couple of fields with the Primary + Secondary (where appropriate) Connection Strings for Blob access to Storage Accounts.

Relevant test:

$ TF_ACC=1 envchain azurerm go test ./azurerm -v -timeout 300m -parallel 5 -run TestAccAzureRMStorageAccount_blobConnectionString
=== RUN   TestAccAzureRMStorageAccount_blobConnectionString
--- PASS: TestAccAzureRMStorageAccount_blobConnectionString (116.37s)
PASS
ok  	github.com/terraform-providers/terraform-provider-azurerm/azurerm	116.397s

Running the rest of the suite now

[tombuildsstuff]

Tests pass:

[grubernaut]

LGTM!

[ghost]

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