record broker time in storage::index_state

[andijcr]

Who is this for and what problem do they have today?

to support moving time based retention to broker time

part of https://github.com/redpanda-data/planning/issues/86

the pr after this, where broker time is actually used, might also offer a workaround for #11922

What are the success criteria?

Why is solving this problem impactful?

Additional notes