Support ALTER TABLE t REMOVE PARTITIONING

[mjonss]

Enhancement

There are no ways to alter a table between partitioned and non-partitioned (unless you renaming tables and using exchange partition to move the data into a partitioned table).

This enhancement is to support ALTER TABLE t REMOVE PARTITIONING.
Also making clear if such operation would allow other DDL operations in the same statement (like changing columns etc.)

Another related enhancement would be to support ALTER TABLE t PARTITION BY ... to (re)partition a table with a new partitioning scheme.

Tasks:

• Implement ALTER TABLE t REMOVE PARTITIONING *: Remove and add partitioning | tidb-test=pr/2125 #42907
• Implement ALTER TABLE t PARTITION BY ... *: Remove and add partitioning | tidb-test=pr/2125 #42907
• Document the new features Added REMOVE PARTITIONING and ALTER TABLE t PARTITION BY docs#13167
• Implement support for the above in TiCDC cdc: support remove and add partitioning tiflow#9670
• Implement support for the above in TiFlash Added support for new DDL actions for remove and add table partitioning tiflash#7822
• Add tests for placement rule *: more tests for add/remove partitioning #47337
• Add tests for auto id ddl: Fix AutoID for Add/Remove partitioning #46909 (Note also affected by Rename table may cause duplicate auto_increment assignment, leading to error #46904)
• Add tests for nonclustered tables
• Add tests for statistics
• Add tests for combining with other features
  • CHECK Constraint
  • ...
• Add tests for tools like BR etc.
• Declare the features GA and update the documentation

Note: FOREIGN Key is not yet supported with Partitioned Tables, so no need to test it, see #46960.