Issue title: Support upserts in concat function
 Issue body: #### Describe your feature request

At the moment the is the existing behaviour in polars for vertical concats is to just append all rows from one dataframe into another. But having a upsert=true parameter would work like doing an upsert in SQL. A 'key' -parameter would most likely be needed as well to specify the column(s) used for calculating the intersect between source and target. So given two dataframes with identical schema, _source_ and _target_, upsert=true would do the following things:

- Append rows from source to target where keys don't match
- Update rows that have the same key in source and target
- Don't append the rows that have the same key and rest of the column values are identical in source and target

This is a common pattern for doing data loads into a persistent table in data warehouses, and it would be awesome if polars could support it.
 Comments: 
 Comment 0: @jkausti  

Great idea.
And may I to know how to append all rows from one dataframe into another at this moment? I just want to insert a new row into a dataframe. Thank you very much.

 Comment 1: > @jkausti
> 
> Great idea. And may I to know how to append all rows from one dataframe into another at this moment? I just want to insert a new row into a dataframe. Thank you very much.

You can use the polars.concat function. So you need to create a dataframe first of the row(s) you want to append to the other dataframe. It is documented here: [https://pola-rs.github.io/polars/py-polars/html/reference/api/polars.concat.html]()
 Comment 2: Thanks @jkausti 
This is Python Polars APIs, but is there any Rust Polars APIs for concat function? I use Rust Polars for my program.
 Comment 3: > Thanks @jkausti This is Python Polars APIs, but is there any Rust Polars APIs for concat function? I use Rust Polars for my program.

@CHCP can you use stackoverflow for that and keep this issue on topic?

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