Retry GraphQL mutations on `SERVICE_UNAVAILABLE` errors

We now have logic to retry all GraphQL reads in the case of a non-successful response, but there are still two issues (https://github.com/github/migration-friction/issues/689 and https://github.com/github/migration-friction/issues/701) that concern the `startRepositoryMigration` mutation. Since @synthead made the necessary change to the monolith to return `SERVICE_UNAVAILABLE` for mutations that have errored but are safe to retry, we should look for that status in the CLI to extend our retry to mutations (specifically `startRepositoryMigration`, which sends the `SERVICE_UNAVAILABLE` error).

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Original comment from @synthead on #666 follows:

>https://github.com/github/github/pull/241010 just shipped to prod! This change makes the Octoshift GraphQL APIs return `"type": "SERVICE_UNAVAILABLE"` in the error payload for failed requests. We can consume this error message when solving for this issue to do informed retries so we're not just running blind :+1:
>
> I made sure in the PR that we only return these errors for requests that can be safely retried. This includes:
>
> - GraphQL queries that could not make a connection to Octoshift
> - GraphQL queries that made a connection to Octoshift, but had a dropped connection while transferring data
> - GraphQL mutations that could not make a connection to Octoshift
>
> Note that I opted to still return a "something went wrong" failure for _mutations_ that had their connections dropped partway because we cannot be confident that server-side transactions haven't been made.