MINOR: stats: introduce stats field ctx

MINOR: stats: introduce stats field ctx

Add a new value in stats ctx: field.
Implement field support in line dumping parent functions
stats_print_proxy_field_json() and stats_dump_proxy_to_buffer().

This will allow child dumping functions to support partial line dumping
when needed. ie: when dumping buffer is exhausted: do a partial send and
wait for a new buffer to finish the dump. Thanks to field ctx, the function
can start dumping where it left off on previous (unterminated) invokation.