ddl refactor: wanna set getPrimaryKey() public for TiCDC to resue it

[crelax]

Enhancement

The function getPrimaryKey() in tidb/ddl is very similar with one function in TiCDC

tidb/ddl/ddl_api.go

Lines 1638 to 1644 in 4ae78cd

	// getPrimaryKey extract the primary key in a table and return `IndexInfo`
	// The returned primary key could be explicit or implicit.
	// If there is no explicit primary key in table,
	// the first UNIQUE INDEX on NOT NULL columns will be the implicit primary key.
	// For more information about implicit primary key, see
	// https://dev.mysql.com/doc/refman/8.0/en/invisible-indexes.html
	func getPrimaryKey(tblInfo *model.TableInfo) *model.IndexInfo {

related issue in TiFlow: pingcap/tiflow#5902

[crelax]

/assign @crelax