feat: Make Table a Mapping

[NickCrews]

Is your feature request related to a problem?

Specifically, I want to be able to to s = ibis.struct(my_table), or {"col1": "default_that_might_be_overwritten", **table, "col2": "always_this_val"}

Describe the solution you'd like

Per #8671 (comment), all that needs to be present is .keys(self) and __getitem__(self, key). So the only thing that is missing is .keys().

This is similar to #8671. See that PR for a few downsides of this approach. Specifically addressing them here:

• this pollutes the namespace, so now if you have a column called "keys" (which honestly seems pretty likely), then if you try to access that column via table.keys you now get an error. You would need to do table["keys"]. This is the reason I didn't add .values() and .items() methods as well.
• To fully follow the collections.abc.Mapping, we would also need to implement __len__(), which I think we all have decided is a footgun and not a good idea.
• That PR suffered from the problem that **StructValue implied that **MapValue and *ArrayValue might work (but they never could). On a Table, we don't suffer from this possible point of confusion.

IDK, maybe these downsides are not worth it, **{c: table[c] for c in table.columns} is not that painful.

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