Up to now, the http driver used quite rudimentary ClickHouse array
detection and parsing that failed to account for single-quoted strings.
As a result, it rendered strings *with the single quotes.* It also would
fail to parse strings that included ClickHouse array brackets.

To address these shortcomings, add proper array parsing to `parser.c`,
based on ClickHouse's own array parsing functions. `ch_http_read_next()`
now detects a field starting with `[` and passes it off to the new
`ch_http_read_array()` function, which detects the start and end of
nested arrays as well as single-quoted strings. The latter it passes off
to `ch_http_read_array_string_literal()`, which converts a ClickHouse
TSV array single-quoted string value to a Postgres double-quoted array
string value.

In all cases, these functions adjust `ch_http_read_state->curpos` to
track position between them; `ch_http_read_next()` no longer uses a
separate `pos` variable.

With these improvements to the TSV parser, remove the naive array
parsing from `char_to_datum()` in `pglink.c`. Also remove the
`VARCHAROID` handling, which seems to have no effect.

Add tests to `http_inserts.sql` to ensure the new functions properly
parse arrays and handle all the special characters. Also update older
tests that incorrectly returned arrays with single quotes; e.g., the
invalid Postgres array `{'num0','num1'}` becomes the correct
`{num0,num1}`.

Test the same values with the binary driver, and fix an issue where it
raised an error for an empty array.

While at it, remove the `buflen` field from `ch_http_read_state`, as its
purpose was overtaken by the introduction of the `StringInfo` in
5c010e5.