Serialisation of client capabilities (take 2)

[pcapriotti]

Fix an oddity in the serialisation of capabilities in the Client structure.

This PR replaces #3873 and simplifies the implementation. No new types are added, and the difference in serialisation is obtained using the Versioned wrapper, which we are already using for versioned changes in serialisation.

The change in the internal API between brig and galley is not backward compatible, so this can cause temporary failures during upgrades. The invariant that brig's clients are a superset of galley's clients will be maintained.

Note: #3904 is a previous attempt at this fix, which overlooked the fact that the Client structure is used in user events, so it was reverted.

https://wearezeta.atlassian.net/browse/WPB-6400

Checklist

• Add a new entry in an appropriate subdirectory of changelog.d
• Read and follow the PR guidelines

[fisx]

it's not a big deal, ignore this if you like. but in general (a) i don't like single-character identifiers, even locally; (b) i want us all to avoid edit wars.

[pcapriotti]

You don't like single-character identifiers, but two-character ones are ok? That's a bit of a strange stance. But sure, I'll revert it.

[mdimjasevic]

@pcapriotti , in an attempt to get the PR through the CI, I took the liberty to rebase the PR on top of develop. I hope you don't mind.

[mdimjasevic]

As far as I can tell, now we're getting actual test failures: https://concourse.ops.zinfra.io/builds/49403780

[pcapriotti]

As far as I can tell, now we're getting actual test failures: https://concourse.ops.zinfra.io/builds/49403780

Thanks for rebasing. Yes, that's right. It should be fixed now.