Serial-console fails for systems in eastus #106

This is for an issue originally opened at Azure/azure-cli#26050.

az serial-console connect fails because the dictionary used for looking up region pairs doesn't have eastus. Official Azure documentation lists the pair for eastus is westus: https://learn.microsoft.com/en-us/azure/reliability/cross-region-replication-azure.

Proposed fix:
_arm_endpoints.py should add this to the dictionary:
'eastus': 'westus',

Error message on with version 0.1.5 of the serial-console extension on Linux:
The command failed with an unexpected error. Here is the traceback:
'eastus'
Traceback (most recent call last):
File "/usr/lib64/az/lib/python3.6/site-packages/knack/cli.py", line 231, in invoke
cmd_result = self.invocation.execute(args)
File "/usr/lib64/az/lib/python3.6/site-packages/azure/cli/core/commands/init.py", line 663, in execute
raise ex
File "/usr/lib64/az/lib/python3.6/site-packages/azure/cli/core/commands/init.py", line 726, in _run_jobs_serially
results.append(self._run_job(expanded_arg, cmd_copy))
File "/usr/lib64/az/lib/python3.6/site-packages/azure/cli/core/commands/init.py", line 697, in _run_job
result = cmd_copy(params)
File "/usr/lib64/az/lib/python3.6/site-packages/azure/cli/core/commands/init.py", line 333, in call
return self.handler(*args, **kwargs)
File "/usr/lib64/az/lib/python3.6/site-packages/azure/cli/core/commands/command_operation.py", line 121, in handler
return op(**command_args)
File "/home/vito/.azure/cliextensions/serial-console/azext_serialconsole/custom.py", line 633, in connect_serialconsole
vm_vmss_name, vmss_instanceid)
File "/home/vito/.azure/cliextensions/serial-console/azext_serialconsole/custom.py", line 597, in check_resource
vmss_instanceid)
File "/home/vito/.azure/cliextensions/serial-console/azext_serialconsole/custom.py", line 735, in get_region_from_storage_account
storage_account_region = get_storage_account_info(storage_account_url, scf)
File "/home/vito/.azure/cliextensions/serial-console/azext_serialconsole/custom.py", line 750, in get_storage_account_info
return ArmEndpoints.region_prefix_pairings[sa_result.location]
KeyError: 'eastus'
To open an issue, please run: 'az feedback'

To reproduce:
az serial-console connect -n vmname -g vmresourcegroup on a VM in the eastus region, with the boot diagnostics storage account set to a custom storage account instead of the default managed storage account, otherwise get_storage_account_info will return None instead of looking up the region_prefix_pairings dictionary:

`def get_storage_account_info(storage_account_url, scf):
from azext_serialconsole._arm_endpoints import ArmEndpoints

if storage_account_url is not None:
storage_account, storage_account_resource_group = parse_storage_account_url(storage_account_url, scf)
if storage_account is not None:
sa_result = scf.storage_accounts.get_properties(storage_account_resource_group, storage_account)
if (sa_result is not None and
sa_result.network_rule_set is not None and
len(sa_result.network_rule_set.ip_rules) > 0):
return ArmEndpoints.region_prefix_pairings[sa_result.location]
return None`