Rename FormatReader to FileOpener

[tustvold]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

Prior to https://github.com/apache/arrow-datafusion/pull/2677/files#diff-0171c6cc11900557bf3bb9bd9fce2a78b8272e7f63ed8259c988ff610334da04L48 there was a trait called FormatReaderOpener. I replaced this with a trait called FormatReader but in hindsight this name is confusing for two reasons:

• It suggests the trait is involved in reading the file, when in reality it is only responsible for creating the reader
• It suggests an instance should exist for each format, when in reality they exist for each FileStream, i.e. partition

Describe the solution you'd like

I would like to rename FormatReader to FileOpener and ReaderFuture to FileOpenFuture

Describe alternatives you've considered

We could not do this

Additional context

This trait came up in discussion on apache/arrow-rs#2230 and I realised its name was unhelpful