Support utf8mb4 character set better for mysql

[Gabriel39]

Description

Although the utf8mb4 character set is more and more popular for MySQL (actually, utf8mb4 becomes default character set already) and maybe utf8mb3 will be removed in future, there are still some problems in current Airflow.
So if you use utf8mb4 as character set, you should set sql_engine_collation_for_ids=utf8mb3_general_ci.

Use case / motivation

In Airflow 2.0, when i use mysql database and utf8mb4, i must set sql_engine_collation_for_ids=utf8mb3_general_ci. Otherwise, the length of primary key of table xcom will exceed the maximum value 3072 bytes.
So I think we should address this problem to avoid more problems in future.

Are you willing to submit a PR?

I'd be willing to

[boring-cyborg]

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[potiuk]

We are not going to do anything about it until MySQL releases a new version. Similarly as in 5.6->5.7 they are likely to increase maximum index size and if they do, the problem will be resolved without any action from our side. This is the plan.