Feature Request: Add support for request_context when executing Online DDL via vtgate

[vkovacik]

Feature Description

Background

We would like to use request_context feature of Online DDL to set an idempotency identifier. Unfortunately request_context is supported only when schema is applied via vtctlclient but not via vtgate.

Request

Add support for request_context when submitting DDL via vtgate. Suggested session variable could be @@ddl_request_context.
This is a follow up of the request raised in PR that implemented request_context.

Use Case(s)

We use Alembic for DB schema migrations. Each migration executes DDL statements via vtgate. The migration may be applied partially only to a subset of the shards. It is currently not possible to retry DDL (via vtgate) to fix the shards that had failed the migration.

[shlomi-noach]

Thank you for the feature request. It makes sense, I will look into it.

[shlomi-noach]

Addressed by #13675