Task: design and add an implementation for a "config:" cursor.

A skeleton of the config cursor exists in the src/cursor/cur_config.c file. It currently implements only "open" and "close" functions. Everything else is either unimplemented or using generic implementation.

Basic requirements (to be done within this ticket):

• The cursor should be able to retrieve numeric values from the current connection config as int64_t.
• The key should be either an enum WT_CONF_ID_* from src/include/conf_keys.h or a full dot-separated path string.
• Boolean values are returned as 0 or 1.
• If a value can't be represented as an integer (e.g. string, list, subconfig), an error should be returned.

Further extensions (if not implemented within this ticket, create additional tickets to implement it):

• Ability to specify the key as either enum or text string.
• Ability to retrieve the values as string.
• Access to lists.
• Access to session-level (and other) configs similarly to what "statistics" cursor does (see __wt_curstat_init() for details).

Note: there are multiple ways to specify the type of the key and of the value - it can be done via a config when opening a cursor or via an additional argument for set_key() and get_value(). Specifying types right at set_key() and get_value() could be less error-prone but might be harder to implement. If different types are not implemented as part of this ticket, the variable arguments to set_key() and get_value() should be designed in advance.