[breaking]: enable clientSegmentCache by default (#84643)
This enables `experimental.clientSegmentCache` by default and the
additional CI resources associated with testing it independently.
There were a few remaining test failures that are individually disabled
with `TODO(client-segment-cache)` markers. We can fix forward as they
are small optimizations rather than anything blocking release.
In a follow-up PR, we'll fully remove the dead code associated with the
`clientSegmentCache: false` case.
