Fix main build against DuckDB Identifier API (#62)

DuckDB's `main` branch (post-1.5) introduced a case-insensitive
`duckdb::Identifier` type and switched catalog/parser/appender APIs to
take it instead of std::string. Its runtime-string constructor is
explicit, so the http_server.cpp call sites passing request-option
strings (CatalogSearchPath::Set, CreateTableInfo, ColumnDefinition,
Catalog::GetCatalog, Appender) no longer compile.

Add an AsCatalogIdentifier() helper that wraps a std::string in an
Identifier when the type is available, and passes the string through
unchanged otherwise. Presence is feature-detected via __has_include of
duckdb/common/identifier.hpp rather than a version check, since `main`
reports a moving dev-suffixed version.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>