Will the key in MapAccess support Express?

The index of an array in ClickHouse can be an function such as :

```sql
string.values[indexOf(string.names, 'endpoint')]='/health'
```

PS: If sqlparser wants to support this syntax,I can submit a PR to support it.

The issue names MapAccess and gives a ClickHouse array-index example, but no file or test. Start by locating MapAccess handling in the Rust SQL parser and checking how array subscripts and function expressions are tested. Done means the shown expression is accepted by the parser with regression coverage.

Written by the indexing model from the issue text.