…rryan)

This PR was merged into the 4.3-dev branch.

Discussion
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Making the serializer configurable by transport

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #30293 (already closed, but I don't think this was reasonably possible before)
| License       | MIT
| Doc PR        | TODO - as many of the new messenger PRs will be done at once

Use cases:
* #30293
* Transport A will be consumed by your Symfony app (so native php serialization is cool) but transport B will be consumed by another app, so you want to serialize as JSON
* Upgrading from Symfony 4.2 to 4.3. The change to the `PhpSerialize` means that messages that were sent to the queue on 4.2, will fail on 4.3. The solution is to use the old serializer in your config. This would allow you to make your existing transport use the old serializer, then migrate to a new transport using the new serializer (then remove the old one later).

Thanks!

Commits
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ef6f23e Making the serializer configurable by transport