Implement EXTRA2_FIELD_DATA_TYPE_INFO in FRM

This task will implement a new frm segment EXTRA2_FIELD_DATA_TYPE_INFO.
Pluggable data types will write their exact data type information in this segment: the data type name and all attributes.

In the standard data type sergment, pluggable data types will still write basic things (like normal fields do) such as:

• The MYSQL_TYPE_XXX replacement type code
• Decimals
• Length
• Collation ID
• Flags

For example, INET6 will later write this information in the standard segment:

• Type code = MYSQL_TYPE_STRING
• Field length = 16
• Collation ID = 63 (my_charset_bin)

and additionally write "inet6" into the new segment EXTRA2_FIELD_DATA_TYPE_INFO.

Only extended data types will write to EXTRA2_FIELD_DATA_TYPE_INFO.
If a table does not have any columns with extended types, EXTRA2_FIELD_DATA_TYPE_INFO won't be written.

Let's use this format for every field:

1. fieldnr, in net_write_length() format
2. data type info length, in net_write_length() format
3. data type info string, according to 2

We don't have extended data types yet, so this task will make sure the new functionality works using some DBUG_EXECUTE_IFs:

• to force some built-in data types write EXTRA2_FIELD_DATA_TYPE_INFO
• to print information about EXTRA2_FIELD_DATA_TYPE_INFO as warnings

This will guarantee that the full cycle is tested:

• writing EXTRA2_FIELD_DATA_TYPE_INFO in build_frm_image()
• parsing EXTRA2_FIELD_DATA_TYPE_INFO in TABLE_SHARE::init_from_binary_frm_image()