Add "lakectl fs upload --recursive" flag

[arielshaqed]

Behaves a lot like s3 cp --recursive: uploads files using their path relative from the local directory.

Example

Suppose you have these files:

/tmp/a/foo
/tmp/a/b/bar
/tmp/a/b/c/d/e/xyzzy

Then lakectl upload --recursive --source /tmp/a/ lakefs://repo@master/top/ will create these objects:

lakefs://repo@master/top/foo
lakefs://repo@master/top/b/bar
lakefs://repo@master/top/b/c/d/e/xyzzy

[arielshaqed]

Alternative solution for https://lakefs.slack.com/archives/C016726JLJW/p1606735311131500.

[arielshaqed]

Alternative to #977, probably don't want both...

[ozkatz]

LGTM!

[arielshaqed]

Thanks! Pulling this one, throwing #977 away.