autoid, ddl: support changing AUTO_INCREMENT attribute to AUTO_RANDOM

[bb7133]

Feature Request

Is your feature request related to a problem? Please describe:
For now, column attribute AUTO_RANDOM is provided by TiDB as a better solution for unique ID allocations.

However, there may be a case where the users have tables that are currently using auto_increment, but the insert volume grows high enough that it becomes a bottleneck in the future.

Describe the feature you'd like:
Allow modifying existing AUTO_INCREMENT columns to AUTO_RANDOM.

Describe alternatives you've considered:
One possible alternative is that, users can create an empty table with the same schema, except that changing AUTO_INCREMENT to AUTO_RANDOM for the newly created table, and then "copy" all data into the new table.

However, this is not easy to use obviously.

Teachability, Documentation, Adoption, Migration Strategy:
Technically this is possible if the AUTO_INCREMENT doesn't consume too much allocation space yet(which should be most of the case).

AUTO_RANDOM can be simply treated as AUTO_INCREMENT with an extra SHARDING part:

| SignPart | ShardingPart | IncrementalPart |

In which the 'SignPart' and 'IncrementalPart' is actually the same with AUTO_INCREMENT, if we treat 'ShardingPart' as all zero for all allocated values for AUTO_INCRMENT column, it can be altered into AUTO_RANDOM attribute.