Remote file operations and driver

[richfitz]

This implements support for remote file operations in a relatively open way.

In contrast with https://github.com/ben-gready/storr.remote/pull/1/files:

• some of the names are changed
• some operations are at a slightly higher level (e.g., delete_file is vectorised and returns success/failure vector)
• some operations are at a slightly lower level, e.g., I opted to implement upload_file and download_file rather than reading RDS directly because aws.s3 is doing this internally any way, and because it simplifies a local cache

All options for driver_rds are honoured and will be checked in the same way (compress, hash_algorithm, etc).

As an example, the helper-remote.R file implements a fake set of operations. This probably calls for a similar automatic testing framework! There are not too many to implement:

• type
• destroy
• create_dir
• list_dir
• exists
• exists_dir
• delete_file
• delete_dir
• upload_file
• download_file
• write_string
• read_string

The upload_file and download_file ones are a bit odd because they take a single directory the destination argument and copy files with the same name always. This is useful once we have the assumption of a local cache, and was also required to implement the ssh driver. Hopefully that's not too restrictive in general.

Still to come:

• can we do bulk uploads more efficiently without requiring it?

Add an optional vectorised operation and use that within storr_remote automatically

[richfitz]

@ben-gready - I don't see how to add you to review this, but you can hopefully make comments at least

[codecov-io]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@2ae8931). Click here to learn what that means. The diff coverage is 100%.

@@            Coverage Diff            @@
##             master      #74   +/-   ##
=========================================
  Coverage          ?   99.27%           
=========================================
  Files             ?       14           
  Lines             ?     1099           
  Branches          ?        0           
=========================================
  Hits              ?     1091           
  Misses            ?        8           
  Partials          ?        0

Impacted Files	Coverage Δ
R/driver_dbi.R	99.65% <ø> (ø)
R/hash.R	100% <100%> (ø)
R/driver_rds.R	100% <100%> (ø)
R/base64.R	100% <100%> (ø)
R/multistorr.R	100% <100%> (ø)
R/storr.R	98.42% <100%> (ø)
R/utils.R	100% <100%> (ø)

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