Support mapping to Postgres JSON type

[theory]

In addition to the JSONB type mapping, add support for the use of the Postgres JSON type mapped to the ClickHouse JSON type. It already worked for the JSON driver, just hadn't been tested. But adding it to the binary driver required a bit more work.

Mainly that work encompasses making the binary type handling functions aware of the Postgres type used in columns. By default, the binary driver supports a short list of types that map to ClickHouse types, but allows for other types by exercising the functions in convert.c to convert between compatible types. These conversion require casts. And while Postgres supports casts between JSON and JSONB, they're COERCION_PATH_COERCEVIAIO casts, which convert.c doesn't handle.

We could consider adding support for these conversions, but since it requires using the _in and _out functions to do the conversion, and since for JSON all we really need is the text version, this seems unnecessarily wasteful in terms of CPU cycles and memory.

Instead, refactor things so that the binary conversion functions are aware of not only explicitly supported type mappings provided by get_corr_postgres_type(), but also the types used for the columns in the foreign tables. Then teach get_corr_postgres_type() to inspect these values in the context of the ClickHouse JSON type and allow both.

For fetches (selects), on the first call to ch_binary_read_row(), populate coltypes with the actual column types and teach make_datum() to examine it when considering JSON types. This requires that the List of attributes be passed to ch_binary_read_row, and also that coltypes be initialized with zero values. Ideally coltypes would be populated by an earlier hook, before fetching starts, but this will do for now.

For updates (inserts), on the other hand, create a new List and populate it with the requisite types in clickhouseBeginForeignInsert and clickhousePlanForeignModify (not sure why it needs both, but it follows the pattern of setting column attribute numbers, which are also in both places). THen pass it as a new argument to the prepare_insert function, now with an updated signature (the HTTP version currently ignores the new argument). Then fetch the types from it in ch_binary_prepare_insert() to pass to get_corr_postgres_type().

Teach init_output_convert_state() not to try to convert between JSON and JSONB types, and add comments to explain what's happening with all this type management stuff.

Finally, update the function and operator pushdown functions to support the -> and ->> operators for JSON, as well as the json_extract_path_text() and json_extract_path() functions. Rename the relevant constants to JSON_* instead of JSONB_*, since they now handle both, and JSON_* is the more generic.

Expand the JSON tests to cover all the same patterns for JSON that were previously covered for JSONB, for both the binary and http drivers. This requires additional alternate expected output files due to minor changes to earlier ClickHouse versions that did not support JSON and returned varying error messages about it.

Document the support for JSON, as well as the operators and functions. In fact, the jsonb_() functions weren't previously documented, so add them, too. Also fix reversed descriptions for the -> and ->> operators and mention that they work for both JSON and JSONB.

While at it, update the exception raised when column_append() can't handle a specific Postgres type to emit the name of the unsupported type.