[SL-3321] [Feature] Allow use dimensional attributes of another model when defining a measure.

### Is this your first time submitting a feature request?

- [x] I have read the [expectations for open source contributors](https://docs.getdbt.com/docs/contributing/oss-expectations)
- [x] I have searched the existing issues, and I could not find an existing issue for this feature
- [x] I am requesting a straightforward extension of existing metricflow functionality, rather than a Big Idea better suited to a discussion

### Describe the feature

I would like to be able to reference dimensional attributes of another model when defining Measures.
This is needed when defining aggregate values that combine fact and dimension tables in dimensional modeling.

Example:

```yaml
semantic_models:
- name: order_fact
model: ref('fct_order')

entities:
- name: order_fact
type: primary
expr: order_key

- name: user_dimension
type: foreign
expr: user_key

measures:
- name: number_of_unique_users
agg: count_distinct
expr: "{{ Dimension('user_dimension__user_id') }}" # Enable this syntax (Encountered an error: Compilation Error Could not render {{ Dimension('user_key__user_id') }}: 'Dimension' is undefined)
```

fct_order:

| order_key | user_key | order_id | order_date | order_price |
| ---------- | -------- | -------- | ------------------- | ----------- |
| 1 | 1 | o1 | 2025-01-01T00:00:00 | 1000 |
| 2 | 2 | o2 | 2025-02-01T00:00:00 | 1200 |
| 3 | 3 | o3 | 2025-03-01T00:00:00 | 800 |

dim_user:

| user_key | user_id | state_code |
| -------- | ------- | ---------- |
| 1 | u1 | NY |
| 2 | u1 | CA |
| 3 | u2 | CA |

### Describe alternatives you've considered

Store `user_id` as a degenerate dimension in the fact table.

Example:

```yaml
semantic_models:
- name: order_fact
model: ref('fct_order')

entities:
- name: order_fact
type: primary
expr: order_key

- name: user_dimension
type: foreign
expr: user_key

measures:
- name: number_of_unique_users
agg: count_distinct
expr: user_id
```

fct_order:

| order_key | user_key | order_id | user_id | order_date | order_price |
| ---------- | -------- | -------- | ------------------- | ----------- | --- |
| 1 | 1 | o1 | u1 | 2025-01-01T00:00:00 | 1000 |
| 2 | 2 | o2 | u1 | 2025-02-01T00:00:00 | 1200 |
| 3 | 3 | o3 | u2 | 2025-03-01T00:00:00 | 800 |

dim_user:

| user_key | user_id | state_code |
| -------- | ------- | ---------- |
| 1 | u1 | NY |
| 2 | u1 | CA |
| 3 | u2 | CA |

Pros:

- Simplifies queries by eliminating joins

Cons:

- Data redundancy: user_id stored in both fact and dimension tables
- Increased storage requirements in fact table
- Violates dimensional modeling best practices

### Who will this benefit?

Data teams implementing dimensional modeling (star schema).

### Are you interested in contributing this feature?

_No response_

### Anything else?

In Looker, it looks like it can be defined as follows.

```yaml
measure: number_of_unique_users {
type: count_distinct
sql: ${dim_user.user_id} ;;
}
```

> [!WARNING]
> I'm not an expert on Looker, so the specification itself or the syntax may be incorrect.

_{[SL-3321](https://linear.app/dbt-labs/issue/SL-3321/[feature]-allow-use-dimensional-attributes-of-another-model-when)}