Add a timeline type for relative time

[emilk]

We currently have two time types:

• Sequence
• Temporal

The problem is that the temporal type is used both for absolute times (2024-01-10 09:44:01Z) and relative times (+32m 12s).
Because of this problem, relative times will show up as 1970-01-01 00:32:12Z in arrow formatting.

Relative times are useful for "time since robot started" or whatever.

I suggest we split Temporal into absolute and relative. The arrow datatype would then become:

• Sequence - > DataType::Int64
• NanosSinceEpoch - > DataType::Timestamp(TimeUnit::Nanosecond
• NanosRelative - > DataType::Duration64(TimeUnit::Nanosecond)

(with reservation for actual names)