[Federation] Partial Queries - Discussion

Something I'd really like to see for federation is to support partial queries/results. As an example:

federated service 1's SDL might be:
```graphql
extend union ConfigTypes = OrgCommunicatonsConfig

type OrgCommunicatonsConfig {
canSendMessages: Boolean!,
canSendSMS: Boolean!
}

extend type Query {
getOrgConfig(): [ConfigTypes!] @partial
}
```

federated service 2's SDL might be:
```graphql
extend union ConfigTypes = OrgSecurityConfig

type OrgSecurityConfig {
setting1: String!,
setting2: String!
}

extend type Query {
getOrgConfig(): [ConfigTypes!] @partial
}
```
The resulting schema would be:

```graphql
union ConfigTypes = OrgSecurityConfig | OrgCommunicatonsConfig

type OrgSecurityConfig {
setting1: String!,
setting2: String!
}

type OrgCommunicatonsConfig {
canSendMessages: Boolean!,
canSendSMS: Boolean!
}

type Query {
getOrgConfig(): [ConfigTypes!]
}
```

The query executor would then forward any query to getOrgConfig to each service and then amalgamate the results so you can then do something like:

```graphql
query {
getOrgConfig() {
... on OrgSecurityConfig {
/// query fields
}
... on OrgCommunicatonsConfig {
/// query fields
}
}
}
```