azurerm_storage_account - adds support for enabling SFTP

[leesutcliffe]

This PR is to add SFTP support to the azurerm_storage_account resource.
A sftp_enabled property has been added which takes a boolean value to enable or disable the service

Relates-to: #14736

Acceptance tests run successfully

make acctests SERVICE=storage TESTARGS='-run=TestAccStorageAccount_isSftpEnabled' TESTTIMEOUT='60m'

==> Checking that code complies with gofmt requirements...
==> Checking that Custom Timeouts are used...
==> Checking that acceptance test packages are used...
TF_ACC=1 go test -v ./internal/services/storage -run=TestAccStorageAccount_isSftpEnabled -timeout 60m -ldflags="-X=github.com/hashicorp/terraform-provider-azurerm/version.ProviderVersion=acc"
=== RUN   TestAccStorageAccount_isSftpEnabled
=== PAUSE TestAccStorageAccount_isSftpEnabled
=== CONT  TestAccStorageAccount_isSftpEnabled
--- PASS: TestAccStorageAccount_isSftpEnabled (190.48s)
PASS
ok      github.com/hashicorp/terraform-provider-azurerm/internal/services/storage       (cached)

[leesutcliffe]

Resubmitting without messing rebase

[github-actions]

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