Provide complete info about byte format of external table field types [DOC84]

[firebird-automations]

Submitted by: @livius2

Plese provide complete info about byte format of supported by external table field types
this info is required to write small and fast external table files by external applications

now in many cases it is created as table with all fields declared as char also for numbers, dates - because it is undocumented
and size and speed of that table is then bad

will be good to see info about supported by external table field types:
-char
-varchar
-smallint
-integer
-bigint
-date
-time
-timestamp