Delimited text format schema definition doesn't work

@vishalaj1 creates an issue for documentation: [https://github.com/MicrosoftDocs/azure-docs/issues/34997](https://github.com/MicrosoftDocs/azure-docs/issues/34997). Duplicating it in this repository as this looks like a bug in ADF rather then in documentation.

> As per the syntax we can define schema.
>
> "schema": [ < physical schema, optional, retrievable during authoring > ],
>
> However, when i define schema:
> "schema": [
> {
> "name": "Tst1",
> "type": "String"
> },
>
> This does not work. The column names are still read as Prop_0 etc.

An expectation are:
- Order of fields in schema is matching column order in delimited text formatted file.
- Data Preview \ WranglingDataFlow \ Import Schema Functionality will use schema definition if this is possible. Logic should look like:
- If "First row as header" is true then extract column names from file and match them with schema to get types
- If "First row as header" is false then extract column names and types from schema based on column order
- If schema is not defined then identify column as "Prop_{column number}" with undefined type. (current logic)