Add IO operations "from/load" to the LazyFrame implementation

[philss]

This is related to #227

In the first PR for that issue, #202, we discussed a possible lazy: true option to control which backend to use. I end up using both :backend and :lazy to make it easier for clients to choose the backend. This adds the possibility for a backend to be implemented both using an eager (the default) and lazy version. WDYT?

I also choose to "delegate" the load of binaries to the eager implementation, and then converting it to lazy. This is mostly for convenience, because there is not such loaders in the Polars backend.

[cigrainger]

I love the direction. Definitely like having both :backend and :lazy opts. Yeah I think that also makes sense regarding loading binaries. It's in memory already anyway so you don't lose much compared to loading from file. Definitely should document that clearly though.

[philss]

Definitely should document that clearly though.

@cigrainger You mean about the :lazy option or the usage of the eager by the load_ functions?
Anyway, I'm going to add documentation in a future PR.