acme.sh Azure DNS does not support Azure Arc endpoints

When using the Managed Identity option (instead of Service Principal), the VM must have rights on the Azure DNS Zone. If this VM is not hosted in Azure, the Instance Metadata Service will be different and will not be able to get credentials needed for it's Managed Identity. For a VM in Azure, it is http://169.254.169.254/metadata/. If the VM is on-premises (for example) and is Arc-enabled so that it's Managed Identity can be used, the Instance Metadata Service is at http://localhost:40342/metadata/. See [this documentation](https://learn.microsoft.com/azure/azure-arc/servers/managed-identity-authentication#acquiring-an-access-token-using-rest-api).

Steps to reproduce
------------------

1. Stand up a virtual machine in any cloud or on-premises and Arc-enable it using [instructions](https://learn.microsoft.com/azure/cloud-adoption-framework/manage/hybrid/server/best-practices/onboard-server-linux).
2. In your Azure DNS Zone, give the Azure Arc resource (representation of the virtual machine which was created in previous step) access to the zone. It should have DNS Zone Contributor access. For general information on how to give a Managed Identity access to Azure, see [these instructions](https://learn.microsoft.com/azure/active-directory/managed-identities-azure-resources/tutorial-linux-vm-access-arm).
3. Export the two variables needed to use Managed Identity: `export AZUREDNS_SUBSCRIPTIONID="";
export AZUREDNS_MANAGEDIDENTITY=true`
4. Run acme.sh using the DNS method: `acme.sh --issue --dns dns_azure --dnssleep 10 --force -d domain.com --staging`

Debug log
-----------------

```
/root/.acme.sh/acme.sh --issue --dns dns_azure --dnssleep 10 --force -d domain.com --staging --debug 2
```
[acme.sh.log](https://github.com/acmesh-official/acme.sh/files/11080233/acme.sh.log)

Potential solution
-----------------
A solution is to modify the `_azure_getaccess_token()` function in dns_azure.sh within the existing code block for ` if [ "$managedIdentity" = true ]; then`.

```
if [ "$managedIdentity" = true ]; then
# https://docs.microsoft.com/en-us/azure/active-directory/managed-identities-azure-resources/how-to-use-vm-token#get-a-token-using-http

# Modification for Arc-enabled server
ChallengeTokenPath=$(curl -s -D - -H Metadata:true "http://localhost:40342/metadata/identity/oauth2/token?api-version=2020-06-01&resource=https%3A%2F%2Fmanagement.azure.com" | grep Www-Authenticate | cut -d "=" -f 2 | tr -d "[:cntrl:]")
ChallengeToken="$(cat $ChallengeTokenPath)"
if [ $? -ne 0 ]; then
echo "Could not retrieve challenge token, double check that this command is run with root privileges."
else
response=$(curl -s -H Metadata:true -H "Authorization: Basic $ChallengeToken" "http://127.0.0.1:40342/metadata/identity/oauth2/token?api-version=2020-06-01&resource=https%3A%2F%2Fmanagement.azure.com")
response="$(echo "$response" | _normalizeJson)"
accesstoken=$(echo "$response" | _egrep_o "\"access_token\":\"[^\"]*\"" | _head_n 1 | cut -d : -f 2 | tr -d \")
expires_on=$(echo "$response" | _egrep_o "\"expires_on\":\"[^\"]*\"" | _head_n 1 | cut -d : -f 2 | tr -d \")
fi
# Modification end
```

I have tested this on my on-premises proxmox server. I did not submit a pull request because I don't know of a good way to check if the machine is running in Azure or if it's Arc-enabled.

Start in `dns_azure.sh`, in the managed-identity block of `_azure_getaccess_token()` where token retrieval is implemented. Compare the current metadata URL path in that block with the Arc-hosted endpoint described in the issue and adjust the logic so both environments can succeed. Reproduce first with the provided `acme.sh --issue --dns dns_azure --dnssleep 10 --force -d domain.com --staging --debug 2` flow, then confirm the issue is done when token acquisition and DNS-01 flow complete successfully for Arc-enabled hosts without regressing normal Azure MI behavior.

索引モデルが issue の本文から書いたものです。