Enforce uniqueness of named_struct field names

[dharanad]

Which issue does this PR close?

Closes #11438

Rationale for this change

What changes are included in this PR?

• Added a check for uniqueness of named struct fields
• add sql logic tests

Are these changes tested?

Tested manually

> select {'A': 1, 'a': 2};
+-----------------------------------------------------+
| named_struct(Utf8("A"),Int64(1),Utf8("a"),Int64(2)) |
+-----------------------------------------------------+
| {A: 1, a: 2}                                        |
+-----------------------------------------------------+
1 row(s) fetched. 
Elapsed 0.007 seconds.


> select {'a': 1, 'b': 2, 'c': 3, 'b': 4};
Execution error: named_struct requires unique field names. Field b is used more than once.

& existing test cases.

Are there any user-facing changes?

[dharanad]

Based on my understanding, since both struct field names and dictionary keys are of type Ident, it seems challenging to distinguish between a NULL value and a 'NULL' string. Folks any any potential workarounds or alternative approaches that might address this issue?

[alamb]

Makes sense to me @dharanad -- thank you 🙏

[alamb]

Thanks again @dharanad