[Index Management] [Data Streams] Be more explicit in the UI about data retention period

[sabarasaba]

This is a follow up of #167006, there are a few things that we could improve to make it more clear for the user what is the current data retention period:

• We might want to consider creating human readable units to display in the table. For example 7m could be interpreted as "minutes" or "months".
• To avoid confusion for the user instead of abbreviating all time units such as "nanos" "micros" etc. it would be better to fully expand it, eg: 3000 microseconds
• To keep our UI consistent to others from kibana, we should add a (non-breaking) space between the number and the unit (30 ns, 4 days, instead of 30ns, 4days).

[elasticmachine]

Pinging @elastic/platform-deployment-management (Team:Deployment Management)

[alisonelizabeth]

If a user selects "Never delete data" and saves, we should represent this in the table view as well (right now this translates as an empty table cell for the "Data retention" column).

I've removed this one and created a separate issue to address: #168494