Support for fread() of file:// URI with spaces

[MichaelChirico]

Closes #7550.

[MichaelChirico]

I also confirm we can't use URLencode() (to avoid special-casing file://)

readLines(URLencode(u))
# Error in file(con, "r") : cannot open the connection
# In addition: Warning message:
# In file(con, "r") :
#   cannot open file '/tmp/RtmpyAROdr/a%20dir/a.csv': No such file or directory

And that https? URIs will not have this issue:

https://stackoverflow.com/a/497972/3576984

I can't quite confirm the situation for ftps? URIs, but it seems like they might support spaces? I'm happy to punt until there's an actual user report, though.

[github-actions]

• HEAD=fread-file-spaces slower P<0.001 for fread disk overhead improved in #6925
  

Generated via commit f2beb63

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Task	Duration
R setup and installing dependencies	2 minutes and 46 seconds
Installing different package versions	21 seconds
Running and plotting the test cases	4 minutes and 3 seconds

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.97%. Comparing base (291a711) to head (f2beb63).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7551   +/-   ##
=======================================
  Coverage   98.97%   98.97%           
=======================================
  Files          87       87           
  Lines       16733    16733           
=======================================
  Hits        16561    16561           
  Misses        172      172           

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