ENH: Allow vector in 'merge_asof' / 'right_on' parameter ('left_on' already accepts it)
#### Is your feature request related to a problem?

`left_on` and `right_on` parameters of [`merge_ordered`](https://pandas.pydata.org/docs/reference/api/pandas.merge_ordered.html) accept vectors of the same length than the DataFrame they relate to.

Although not stated in the documentation, `left_on` parameter of [`merge_asof`](https://pandas.pydata.org/docs/reference/api/pandas.merge_asof.html) also accept vector of the same length than the left DataFrame.

But not `right_on`. This would allow not having to recreate a DataFrame with a new column for the keys in case it is not already part of the right DataFrame.

#### Describe the solution you'd like

`right_on` to accept vector as well.

#### API breaking implications

`right_on` will be able to accept vector.

#### Describe alternatives you've considered

As a workaround, I will create a new DataFrame with a new column to embed the keys.
