[BUG] [Azure.Identity] SharedTokenCacheCredential throws error "Multiple accounts matching the specified..."

[erich-wang]

Describe the bug
Please provide the description of issue you're seeing.

This is based on the issue Azure/azure-powershell#14023 reported by Azure PowerShell user.

If there're multi tenants for one account, when calling SharedTokenCacheCredential.GetToken with tenantId and _skipTenantValiation as true, we'll get error that "Multiple accounts matching the specified ...", it looks like a bug at
https://github.com/Azure/azure-sdk-for-net/blob/master/sdk/identity/Azure.Identity/src/SharedTokenCacheCredential.cs#L148-L159.
We probably need to change the logic to:

            // filter the accounts to those matching the specified user and tenant
            List<IAccount> filteredAccounts = accounts.Where(a =>
                // if _username is specified it must match the account
                ((string.IsNullOrEmpty(_username) || string.Compare(a.Username, _username, StringComparison.OrdinalIgnoreCase) == 0))
                &&
                // if _tenantId is specified it must match the account
                (string.IsNullOrEmpty(_tenantId) || string.Compare(a.HomeAccountId?.TenantId, _tenantId, StringComparison.OrdinalIgnoreCase) == 0)
            ).ToList();

            if (filteredAccounts.Count == 0 && _skipTenantValidation)
            {
                filteredAccounts = accounts.Where(a =>
                    // if _username is specified it must match the account
                    (string.IsNullOrEmpty(_username) || string.Compare(a.Username, _username, StringComparison.OrdinalIgnoreCase) == 0)
                ).ToList();
            }

            if (filteredAccounts.Count != 1)
            {
                throw new CredentialUnavailableException(GetCredentialUnavailableMessage(accounts, filteredAccounts));
            }

Expected behavior
What is the expected behavior?

Actual behavior (include Exception or Stack Trace)
What is the actual behavior?

Get the following error:

DEBUG: SharedTokenCacheCredential.GetToken invoked. Scopes: [ https://management.core.windows.net//.default ]
ParentRequestId:
DEBUG: SharedTokenCacheCredential.GetToken was unable to retrieve an access token. Scopes: [
https://management.core.windows.net//.default ] ParentRequestId: Exception:
Azure.Identity.CredentialUnavailableException (0x80131500): SharedTokenCacheCredential authentication unavailable.
Multiple accounts matching the specified username: ***@ntdconsulting.com tenantId: xxx-03bf-480b-b436-5df551f9f911
were found in the cache.

To Reproduce
Steps to reproduce the behavior (include a code snippet, screenshot, or any additional information that might help us reproduce the issue)

NA

Environment:

• Name and version of the Library package used: [Azure.Identity 1.4.0-beta.1]
• Hosting platform or OS and .NET runtime version (dotnet --info output for .NET Core projects): [e.g. Azure AppService or Windows 10 .NET Framework 4.8]
• IDE and version : [e.g. Visual Studio 16.3]