pubsublite: use of 10GB int consts breaks 32 bit systems

[hongalex]

697s # cloud.google.com/go/pubsublite/pscompat
697s src/cloud.google.com/go/pubsublite/pscompat/settings.go:125:21: cannot use 1e10 (untyped float constant 1e+10) as int value in struct literal (truncated)

and

# cloud.google.com/go/pubsublite/internal/wire [cloud.google.com/go/pubsublite/internal/wire.test]
src/cloud.google.com/go/pubsublite/internal/wire/settings_test.go:41:34: cannot use 1e10 (untyped float constant 1e+10) as int value in assignment (truncated)
src/cloud.google.com/go/pubsublite/internal/wire/settings_test.go:136:36: cannot use 1e10 (untyped float constant 1e+10) as int value in assignment (truncated)