Allow defining indexes over JSON complex type columns

[roji]

This refers to defining an index on the table column (e.g. the nvarchar column in SQL Server, or the new JSON type column), not an index over JSON properties inside the document.

Depends on:

• Allow specifying more facets on JSON column #28591

[ErikEJ]

You cannot define indexes against wide string columns in SQL Server.

[roji]

@ErikEJ that is why this issue depends on #28591 - though note that other databases do not necessarily have this limitation.

[roji]

Note that for SQL Server, a new JSON type is being introduced, with dedicated indexing capabilities probably coming too. We should definitely wait to see where things land there before doing anything here.

[roji]

Note npgsql/efcore.pg#3371 where this is blocking GIN index definition over JSON columns (fully supported in PostgreSQL).

With the focus switch to complex types, this issue should track that now.