Feature Request: add DDL strategy flag linter

[shlomi-noach]

Feature Description

Today, DDL strategy (whether --ddl-strategy flag in vtctldclient ApplySchema or @@ddl_strategy session variable), accepts a strategy and some flags. For example: vitess --postpone-completion.

Due to historical reasons, vitess parses the flags it knows about, and ignores the flags it does not know about, or rather, it sends them to the underlying schema change tool in the hope it knows what to do with them.

This is still the case, as it's possible to run --ddl-strategy="gh-ost --max-load=Threads_running=100".

But when it comes to the vitess (aka online) strategy, we can and should do a better job at linting the flags. Today, it's possible to enter vitess --ppstpone-completon, which is a typo. Vitess will silently ignore the flag. Instead, it should error.

Use Case(s)

Running Online DDL with strategy flags.