fix: Make supporting classes of AwsCredentials serializable

[jmahonin]

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Fixes #1112 ☕️

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[jmahonin]

As with #1111 the CLA was signed yesterday for a workgroup jmahonin@decodable.co is a member of. Perhaps waiting for propagation delay.
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[davidrabinowitz]

This can fix GoogleCloudDataproc/spark-bigquery-connector#845 as well. @jmahonin Do you need help with it?

[jmahonin]

This can fix GoogleCloudDataproc/spark-bigquery-connector#845 as well. @jmahonin Do you need help with it?

Just a review at this point!

I can confirm this PR fixes the same serialization issue I observed with Flink. I'm not in a position to test against Spark, but based on past experience, I likewise suspect this will fix it there as well.

[davidrabinowitz]

@jmahonin That's fine, I'll test it once released. Can you please rebase the PR so it can be merged?

[jmahonin]

Will do, expect a ping on Monday

[jmahonin]

@davidrabinowitz Rebased

[TimurSadykov]

Please add serialize unit tests for affected classes. You can leverage existing helper for it by extending the BaseSerializationTest

[jmahonin]

Good call @TimurSadykov , added the tests and discovered ExternalAccountCredentials.ServiceAccountImpersonationOptions was not serializable. Added that as well.

There was already a serialization test on ImpersonatedCredentials, though they all use a mock transport factory so there isn't coverage on instantiating the factory in readObject (which is true of the existing serialization tests as well).

I didn't add a specific test for the deserialization of SystemEnvironmentProvider, it is similarly stubbed out with a TestEnvironmentProvider in the tests.

[jmahonin]

I've updated the unit tests to ensure subclasses of ExternalAccountCredentials have similar serialize() tests.

However, the PluggableAuthCredentials uses a functional interface PluggableAuthHandler, which aren't easily serializable. Given the intent appears to be retrieving tokens using scripts / executables, it's not clear to me if this credential source should be serializable at all. Please advise.

[TimurSadykov]

LGTM

[jmahonin]

This fell off my radar for a bit. I see there's an approval and a request for changes re: subclasses having their own serialVersionUIDs. I'm happy to make the change, but want to confirm first given the PR's been approved.

[lsirac]

jmahonin please make the change and we will merge the PR, LG otherwise

[jmahonin]

jmahonin please make the change and we will merge the PR, LG otherwise

This is done. I didn't include a serialVersionUID for PluggableAuthCredentials for the same reason described above here
#1113 (comment)