Add `structs.merge`

[mattyclarkson]

one = struct(a = 1)
two = struct(b = 2)
overwrite = struct(a = 3)
merged = structs.merge(one, two , overwrite)
# struct(a = 3, b = 2)

[mattyclarkson]

I am unclear why this is failing the buildifier job in the CI.

buildifier (same version as CI) is clean for me locally on lib/structs.bzl, tests/structs_test.bzl, MODULE.bazel and WORKSPACE which have been modified.

Maybe an infra-failure?

[mattyclarkson]

I learnt how to read the BuildKite annotations tab 😊 buildifier should be fixed.

[mattyclarkson]

@fmeum this is good to go.

[fmeum]

@mattyclarkson I don't have write access, I can just help you get this in your shape for review by Google folks