feat(api): Add `exists` flag to attribute filter

### Description
Add an `exists` field to the `EntityAttributeFilter` input object

**Schema**
```graphql
input EntityAttributeFilter {
attribute: String!
exists: Boolean! = true # <---------- NEW
value: String
valueNot: String
valueIn: [String!]
valueNotIn: [String!]
valueType: ValueType
valueTypeNot: ValueType
valueTypeIn: [ValueType!]
valueTypeNotIn: [ValueType!]
}
```

**Example Query**
```graphql
query {
entities(
# Select entities for which the name attribute ("LuBWqZAu6pz54eiJS5mLv8") exists
where: {
attributes: [
{attribute: "LuBWqZAu6pz54eiJS5mLv8", exists: true}
]
},
spaceId: "25omwWh6HYgeRQKCaSpVpa",
first: 10
) {
id
...
}
}
```

### Implementation checklist
- [ ] Update the `PropFilter` struct in the `grc20-core/src/mapping/query_utils/prop_filter.rs` module
- [ ] Add the `exists: Option` field to the struct
- [ ] Update the `PropFilter::subquery` method to apply the an `{field} IS NOT null` or `{field} IS null` filter (if it is not `None`) to the where clause **only if** no other filter have been applied.
- [ ] Update the `EntityAttributeFilter` struct in the `api/src/schema/attribute_filter.rs` module
- [ ] Add the `exists` field to the struct
- [ ] Update the `impl From for mapping::AttributeFilter` block
- [ ] Update the `EntityAttributeFilter::value_filter` method to set the new `exists` field on the `PropFilter`
- [ ] Test!

The work is in two files: grc20-core/src/mapping/query_utils/prop_filter.rs and api/src/schema/attribute_filter.rs. Start by reading the PropFilter struct and its subquery method to understand how filters are applied. Then update the EntityAttributeFilter struct and its conversion to mapping::AttributeFilter. The task is to add an exists flag that adds an IS NOT null or IS null condition to the SQL where clause only when no other filters are applied. Testing will involve verifying the GraphQL query works as shown.

Written by the indexing model from the issue text.