On-conflict support for partial unique indexes

I was trying to add Hasura to an existing project and noticed that my Postgresql Unique Indices weren't showing up in the Hasura Console.

These Postgresql Unique Indices are also missing in the **on_conflict constraint enum** in the GraphQL

**NOTE**: I am not using Hasura to manage my db migrations.

Docker Image: **hasura/graphql-engine:v1.0.0.cli-migrations**

GraphQL query:
```gql
mutation {
insert_MyTable(
on_conflict: {
constraint: MyTable_column1_column2_key,
update_columns: [ column3, column4 ]
},
objects: {
column1: "value1",
column2: "value2",
column3: "value3",
column4: "value4"
}
) {
returning {
id
}
}
}
```
Responds with the following error:
```json
{
"errors": [
{
"extensions": {
"path": "$.selectionSet.insert_MyTable.args.on_conflict.constraint",
"code": "validation-failed"
},
"message": "unexpected value \"MyTable_column1_column2_key\" for enum: 'MyTable_constraint'"
}
]
}
```