Fork `arrow2` and get rid of `polars`

[emilk]

While we want to migrate from the arrow2 crate to arrow (#3741), it is a big task that we would rather punt on right now. It is technical debt, but the debt is not going to grow significantly. The gains don't justify the potential rabbit hole of paint it could turn into.

One of the major reasons to migrate away from arrow2 is because DataType has a huge overhead, especially when cloned.

We have a PR to fix it (jorgecarleitao/arrow2#1469) but it is unmerged, because arrow2 in unmaintained.

So: we fork arrow2 as re_arrow2, merge our PR, and solve our immediate memory issue.

Since polars require arrow2, we need to stop using it. We only have it for a few tests.

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We should revisit the migrating away from arrow2 when we start exposing arrow things to the users (e.g. support data queries in the SDK) and/or when we want to interface with a some data frame crate.

[emilk]

We should replace our polars use with our own DataTable

[kylebarron]

fwiw, Polars also forked arrow2 into polars-arrow, and I don't think it has a dependency on arrow2 anymore

[emilk]

We want to get rid of polars anyhow because of its huge dependency tree

[emilk]

Removing polars blocked on landing caching to avoid conflicts