feat: sql handlers for custom media types

* test text/html and drop HtmlRawOutputSpec.hs
* all tests passing, removed all pendingWith
* make functions compatible with pg <= 12
* move custom media types tests to own spec
* anyelement aggregate
* apply aggregates without a final function
* overriding works
* overriding anyelement with particular agg
* cannot override vendored media types
* plan spec works with custom aggregate
* renamed media types to make clear which ones are overridable
* correct content negotiation with same weight
* text/tab-separated-values media type
* text/csv with BOM plus content-disposition header

postgrest.cabal

@@ -204,11 +204,11 @@ test-suite spec
                       Feature.OptionsSpec
                       Feature.Query.AndOrParamsSpec
                       Feature.Query.ComputedRelsSpec
+                      Feature.Query.CustomMediaSpec
                       Feature.Query.DeleteSpec
                       Feature.Query.EmbedDisambiguationSpec
                       Feature.Query.EmbedInnerJoinSpec
                       Feature.Query.ErrorSpec
-                      Feature.Query.HtmlRawOutputSpec
                       Feature.Query.InsertSpec
                       Feature.Query.JsonOperatorSpec
                       Feature.Query.MultipleSchemaSpec