Datafusion substrait does not set 'maintain_singular_struct' in MaskExpression

### Describe the bug

Datafusion substrait does not set 'maintain_singular_struct' in MaskExpression

https://github.com/substrait-io/substrait/blob/aab01537d750f6fb296a7fe8e18299d04ecc7a83/proto/substrait/algebra.proto#L1298-L1300

### To Reproduce

Generate the substrait plan from the following SQL

SQL
```
SELECT PS_AVAILQTY
FROM 'partsupp'
ORDER BY PS_AVAILQTY
```

The following substrait plan will be generated:

```
{
"relations": [
{
"root": {
"input": {
"fetch": {
"input": {
"sort": {
"input": {
"read": {
"baseSchema": {
"names": [
"ps_partkey",
"ps_suppkey",
"ps_availqty",
"ps_supplycost",
"ps_comment"
]
},
"projection": {
"select": {
"structItems": [
{
"field": 2
}
]
}
},
"namedTable": {
"names": [
"partsupp"
]
}
}
},
"sorts": [
{
"expr": {
"selection": {
"directReference": {
"structField": {}
}
}
},
"direction": "SORT_DIRECTION_ASC_NULLS_LAST"
}
]
}
},
"count": "10"
}
},
"names": [
"ps_availqty"
]
}
}
],
"version": {
"minorNumber": 51,
"producer": "datafusion"
}
}
```

projection doesn't include 'maintain_singular_struct' and therefore fails when running the plan through the substrait-validator
```
"projection": {
"select": {
"structItems": [
{
"field": 2
}
]
}
},
```

### Expected behavior

_No response_

### Additional context

_No response_

Start by reproducing the SQL query and inspecting the generated Substrait plan's projection, especially the select structItems shown in the issue. Compare that projection with the Substrait MaskExpression definition linked in the report and validate the corrected plan with substrait-validator; done means the plan is accepted.

Written by the indexing model from the issue text.