Add kv_list and refactor session_settings with it

[theory]

Compare with #95.

A new structure, kv_list, represents a list of key/value pairs. It does not use an array of structs for each pair, because a custom GUC value must be allocated as a single block,

The kv_list constructor takes a Postgres List argument, iterates over the keys and values, and then allocates a block of memory for them (with options to use palloc, malloc, or guc_malloc (which falls back on malloc prior to Postgrs 16)). It then iterates over them again to pack them into the memory space beyond the kv_list struct itself. The design follows the precedent of the ConvertTimeZoneAbbrevs() function in Postgres's datetime.c.

Use it in chfdw_check_settings_guc() to create a kv_list from the List returned by chfdw_parse_options() and assign it to **extra. Postgres hands this pointer to chfdw_settings_assign_hook(), where we assign it to the ch_session_settings_list global, which can then be fetched via chfdw_get_session_settings().

Of course this would be a tricky interface to iterate manually, so also add another struct, kv_iter, that handles the logic of iterating through the keys and values in a kv_list. Since it sets up the values for each iteration of the loop, the logic using it is simpler than the previous use of List, as seen in the http and binary engines.

While at it, cChange chfdw_parse_options() to use pstrdup() instead of strdup, since its memory is handled by a Postgres memory context (and surely was leaking before).

[serprex]

I prefer this to #95 but see merit of both