Type converters, comparers etc. cannot be supported if they contain expressions incompatible with expresion tree

Following fixes in #32659, Npgsql's NpgsqlArrayConverter can now be successfully translated from LINQ to C#. However, that still doesn't work with the compiled model, since the value converter accepts an `Expression>` parameter, but the LINQ tree contains statements and loops, which are invalid there. This means that converters/comparers can currently only be used if they're restricted to the set of nodes that are valid inside C# expression trees.

A possible way around this is to allow value converters to be constructed without an expression tree; for example, implementations would simply extend ValueConverter and override regular methods as usual. Then, the materializer would then check whether an expression is tree is available, and if not, it would fall back to calling the delegate from the tree rather than integrating an expression tree inline.

Another alternative is to specifically detect this case in LinqToCSharpSyntaxTranslator, and to emit C# code creating the LINQ expression nodes, rather than the C# code directly ("quoting" them). So instead of emitting `1 == 2`, we'd emit `Expression.Binary(Expression.Constant(1), Expression.Constant(2))`. We don't currently have a need for quoting elsewhere, and this is quite a heavy approach.