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!