backfill experiments

[Hona]

This would close #1435

To enable back-filling events into Marten, I'm having a play with some APIs to enable that, its very experimental, but open to feedback

example consumption:

session
    .Events.StartStream(player3, playerRegistered3)
    .WithBackfill(
        created: DateTimeOffset.FromUnixTimeSeconds(576329),
        timestamp: DateTimeOffset.FromUnixTimeSeconds(576329)
    );

I don't like the below syntax, it would be much nicer to also use .WithBackfill - however the StreamAction seems not 1->1 with events.

session.Events.Append(map1, DateTimeOffset.FromUnixTimeSeconds(646329), record4);
session.Events.Append(map1, DateTimeOffset.FromUnixTimeSeconds(656329), record5);

[jeremydmiller]

@Hona Hey man, can you fix up the white space issues on this? I don't even know what it is you were trying to do here

[Hona]

Heya, sure @jeremydmiller I can clean up the PR for you.

The main thing I wanted to do a PoC for was setting custom date times (to convert standard data into an event stream historically)

e.g. of important code from the PR is https://github.com/JasperFx/marten/pull/3358/files#diff-28a99daf7298c7f4f19399310e0b9fbbb0ce7596493d50038109fc2cda5b845cR115-R385