Implement `s3am get-sse-keys URL /PATH/TO/OUTPUT`

[hannes-ucsc]

URL should be enforced to be s3://BUCKET/ or s3://BUCKET/KEY where KEY is to be interpreted as a key prefix if --prefix is given (to be consistent with s3am upload). All matching objects on S3 should be listed and the master key should be used to derive a per-file key. The resulting per-file key and the object's URL should be written to the output file, one line per object in the form s3://BUCKET/FULL/KEY\tSSE_KEY_BASE64 where SSE_KEY_BASE64 is the base64 encoding of the binary SSE key and \t is the tab character.

The master key should be specified using --sse-key and friends but those options should be mandatory for get-sse-keys, despite the oxymoronic nature of that requirement.

[arkal]

I'll get to this tomorrow!

[arkal]

I suggest there be 4 options for URL

1. S3://BUCKET/ - All keys in the bucket
2. S3://BUCKET/KEY/ - All keys in the bucket in folder KEY (must point to a folder)
3. S3://BUCKET/KEY - single KEY in the bucket (must point to a file)
4. S3://BUCKET/KEY + --prefix - All keys in the bucket in with prefix KEY

There is a small chance that the user may have something like

UCSC/
UCSC/file_1
UCSC/file_2
UCSC_manifest

And if we use your suggestion, saying S3://BUCKET/UCSC --prefix gives you all 4.
Using this suggestion, you can get all files in UCSC with S3://BUCKET/UCSC/

Might be worth considering?