…n for microbatch models

Upon the initial implementation of microbatch models, the the `start` for a batch was _optional_.
However, in c3d87b8 they became guaranteed. Thus the if statement
guarding when `start/end` isn't present for microbatch models was no longer actually doing anything.
Hence, the if statement was safe to remove.

This is temporary as a POC. In the end, sample mode can't depend on the arguments
`--event-time-start/end` and will need to be split into their own CLI args / project
config, something like `--sample-window`. The issue with using `--event-time-start/end`
is that if people set those in the project configs, then their microbatch models would
_always_ run with those values even outside of sample mode. Despite that, this is a
useful checkpoint even though it will go away.

…e-start/end`

Using `--event-time-start/end` for sample mode was conflicting with microbatch models
when _not_ running in sample mode. We will have to do _slightly_ more work to plumb
this new way of specifying sample time to microbatch models.

…dule

This is mostly symbolic. We are going to be adding some utilities for "event_time"
type things, which will all live in the `event_time` submodule. Additionally we plan
to refactor `/incremental/materializations/microbatch.py` into the sub module as well.

The `MicrobatchBuilder.offset_timestamp` _truncates_ the timestamp before
offsetting it. We don't want to do that, we want to offset the "raw" timestamp.
We could have split renamed the microbatch builder function name to
`truncate_and_offset_timestamp` and separated the offset logic into a separate
abstract function. However, the offset logic in the MicrobatchBuilder context
depends on the truncation. We might later on be able to refactor the Microbatch
provided function by instead truncating _after_ offsetting instead of before.
But that is out of scope for this initial work, and we should instead revisit it
later.

The previous commit began using a submodule of the dateutil builtin
python library. We weren't previously using this library, and thus didn't
need the type stubs for it. But now that we do use it, we need to have
the type stubs during development.

In most cases people will want to set "relative" sample windows, i.e.
"3 days" to sample the last three days. However, there are some cases
where people will want to "specific" sample windows for some chunk of
historic time, i.e. `{'start': '2024-01-01', 'end': '2024-01-31'}`.

… changes