[Python][Substrait] Acero consumer is unable to consume count function from substrait query plan

SQL
```java

SELECT
o_orderpriority,
count(*) AS order_count
FROM
orders
GROUP BY
o_orderpriority
```
The substrait plan generated from SQL, using Isthmus.

substrait count: 

Running the substrait plan with Acero returns this error:
```java

E   pyarrow.lib.ArrowInvalid: JsonToBinaryStream returned INVALID_ARGUMENT:(relations[0].root.input.aggregate.measures[0].measure) arguments: Cannot find field.  
```
 

From substrait query plan:

relations[0].root.input.aggregate.measures[0].measure
```java

"measure": {
"functionReference": 0,
"args": [],
"sorts": [],
"phase": "AGGREGATION_PHASE_INITIAL_TO_RESULT",
"outputType": {
"i64": {
"typeVariationReference": 0,
"nullability": "NULLABILITY_REQUIRED"
}
},
"invocation": "AGGREGATION_INVOCATION_ALL",
"arguments": []
}
```
```java

"extensions": [{
"extensionFunction": {
"extensionUriReference": 1,
"functionAnchor": 0,
"name": "count:opt"
}
}],
```
Count is a unary function and should be consumable, but isn't in this case.

**Reporter**: [Richard Tia](https://issues.apache.org/jira/browse/ARROW-17061) / @richtia
#### PRs and other links:
- [GitHub Pull Request #14123](https://github.com/apache/arrow/pull/14123)

_{**Note**: *This issue was originally created as [ARROW-17061](https://issues.apache.org/jira/browse/ARROW-17061). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Reproduce the SQL aggregation with the shown Substrait plan and inspect Acero's Substrait plan conversion around relations[0].root.input.aggregate.measures[0].measure. Compare the count:opt extension and its zero-argument measure with the consumer's expected representation. Done means the plan is accepted and the grouped count query executes without the Cannot find field error; PR #14123 is also relevant.

Written by the indexing model from the issue text.