KeyVault: Add Operation to List Private Endpoint Connections

The Azure KeyVault API supports retrieval of a specific private endpoint connect (by name), but does not support listing all private endpoint connections by Vault.

In scenarios where the _**name**_ of the private endpoint connection cannot be determined (but attributes can), the ability to list private endpoint connections is valuable.

An example spec would look something like this:

```json
"/subscriptions/{subscriptionId}/resourceGroups/{resourceGroupName}/providers/Microsoft.KeyVault/vaults/{vaultName}/privateEndpointConnections": {
"get": {
"tags": [
"PrivateEndpointConnections"
],
"description": "List all private endpoint connections on a key vault.",
"operationId": "PrivateEndpointConnections_List",
"parameters": [
{
"$ref": "#/parameters/SubscriptionIdParameter"
},
{
"$ref": "#/parameters/ResourceGroupName"
},
{
"$ref": "#/parameters/VaultName"
},
{
"$ref": "#/parameters/ApiVersionParameter"
}
],
"responses": {
"200": {
"description": "Successfully retrieved private endpoint connections.",
"schema": {
"$ref": "#/definitions/PrivateEndpointConnectionListResult"
}
}
}
}
},
```

Locate the Azure Key Vault REST API specification and compare the existing private endpoint connection operation with the requested collection route. Add the list operation with the shown parameters and response shape, then validate the specification using the repository's standard checks.

Written by the indexing model from the issue text.