Add datetime type to param.Value

[andreasjansson]

When filtering on creation time in replicate ls --filter "started >= 2020-11-12 we treat the time value 2020-11-12 as a string and then use a special case to convert started to a timestamp. It would be more robust to actually have a datetime param.Value type.

[gan3sh500]

@andreasjansson Can I take this up? I'm trying to get familiar with the go part of the codebase and learn go. It seems like a simple enough place to start.

[bfirsh]

@gan3sh500 Go for it! No need to ask permission, but probably worth commenting to say you're working on something. We'll assign issues to ourselves if we're working on them. :)

You might want to base on top of #408, or at least be aware of the changes in case they collide with yours.

[gan3sh500]

@bfirsh Okay. Just checking if anyone else was working on it. I'll take a look at #408 first.