MDEV-11245 Move prepare_create_field and sp_prepare_create_field() as methods to Column_definition

Some upcoming tasks, e.g.:
- MDEV-10577 sql_mode=ORACLE: %TYPE in variable declarations
- MDEV-10914 ROW data type for stored routine variables

will need to reuse the code implemented in prepare_create_field(),
sp_prepare_create_field(), prepare_blob_field().

Before reusing this code, it's a good idea to move these global functions
as methods to Column_definition.

This patch:
- actually moves prepare_create_field(), sp_prepare_create_field(),
  prepare_blob_field() as methods to Column_definition
- makes sp_prepare_create_field() call prepare_create_field() at the end,
  to avoid duplicate code in MDEV-10577 and MDEV-10914.
- changes the return data type for prepare_create_field() from int to bool,
  to make it consistent with all other functions returning "ok" or "error".
- moves the implementation sp_head::fill_field_definition() from sp_head.cc
  to sp_head.h, as it now uses globally visible Column_definition methods,
  and is very simple, so inlining is now possible.
- removes the unused "LEX*" argument from sp_head::fill_field_definition()

sql/sql_table.h

@@ -251,11 +251,6 @@ bool quick_rm_table(THD *thd, handlerton *base, const char *db,
                     const char *table_name, uint flags,
                     const char *table_path=0);
 void close_cached_table(THD *thd, TABLE *table);
-bool sp_prepare_create_field(THD *thd, MEM_ROOT *mem_root,
-                             Column_definition *sql_field);
-int prepare_create_field(Column_definition *sql_field,
-			 uint *blob_columns,
-			 longlong table_flags);
 CHARSET_INFO* get_sql_field_charset(Create_field *sql_field,
                                     HA_CREATE_INFO *create_info);
 bool mysql_write_frm(ALTER_PARTITION_PARAM_TYPE *lpt, uint flags);