flowinfra: preserve flowRetryableError correctly across network

[yuzefovich]

This commit makes it so that flowinfra.flowRetryableError type is
correctly preserved across network. Previously, if the error originated
on the remote node, the coordinator node would receive
errbase.opaqueLeaf error since the decoder method wasn't registered
for the error, now the error is preserved correctly.

Release note: None

[cockroach-teamcity]

This change is 

[yuzefovich]

I found this when working on #82752, here is how the error looks during TestChangefeedHandlesDrainingNodes:

[yuzefovich]

Fixed the linter.

TFTR!

bors r+

[craig]

Build failed (retrying...):

• GitHub CI (Cockroach)

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)

[blathers-crl]

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
3. The backport branch contained merge commits.

You might need to create your backport manually using the backport tool.

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error creating merge commit from 91acca3 to blathers/backport-release-21.2-85500: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 21.2.x failed. See errors above.

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