WL#1279: Add table description to xml dump

Affects: Server-7.1   —   Status: Code-Review   —   Priority: Medium

Display a table description in xml output the same way as 'describe table;' 
command does it.
For example
for create table t1(a int, key(a));

...
<table_description>
  <field_description>
     <Field>a</Filed>
     <Type>int(11)</Type>
     <Null>YES</Null>
     <Key>MUL</Key>
     <Default>NULL</Default>
     <Extra></Extra>
  </field_description>
</table_description>

or
<table_description>
  <field_description Field="a" Type="int(11)" Null="YES"...> 
</field_description>