Add GraphQL endpoint to expose available group and instance templates
### Problem
Backend part of https://gitlab.com/gitlab-org/gitlab/-/issues/442350#note_1810798716+.
We expose available list of instance and group templates via Rails controller. This approach doesn't support a pagination and should be replaced with a separate GraphQL endpoint.
### Proposal
1. Add a GraphQL endpoint to fetch instance templates
2. Add a GraphQL endpoint to fetch group templates
An example of the response
```graphql
{
user(username:"user") {
id
groupTemplates {
nodes {
id
name
description
projects {
nodes {
id
name
description
}
}
}
}
instanceTemplates {
nodes {
id
name
description
}
}
}
}
```
issue