Make extracting information from execution plans easier in the code and reduce the repeated code

[time-and-fate]

Enhancement

Currently, in tidb code, when we want to get some information from an execution plan, we have to implement a method for the planner/core.Plan type. Usually, we have to iterate through the tree in the method. And sometimes, we will collect some basic information in different places.

This causes some unnecessary repeated code and some bugs:
#28731 Generating the execution plan text from a Plan is repeated for the EXPLAIN statement, statements summary, and slow log/slow query. And their results are different.
#35687 The logic of iterating the plan tree is updated because of new operators, but only for EXPLAIN statements. The logic for stmt summary and the slow query/log is missed.

And we are still adding more methods to extract different information from Plan in different places. So it's better if we can make it more convenient and reduce repeated code.

A possible solution: #28731 (comment)