make heavy dependency optional

[kripnerl]

Closes #280

• Divide S3 and mongo dependencies into eventual groups.
• Remove obsolete dependencies.
• Add installation instructions.

[kripnerl]

By the way, I did quite an ugly hack with tests to run. I'm not sure whether to leave it like this or find some more elegant solution.

If there are pytest I would add some flags to run some particular tests only if the selected package is installed. To be sure, that the library is not imported where it shouldn't be.

Nevertheless, I would appreciate this request to be merged as soon as possible since the dependency hell, which is (most likely) botocore causing is blocking me quite a lot - poetry lock file is being calculated for over several minutes. Otherwise it takes only few seconds.

[orso82]

I don't think s3 and mongo are used much, if at all. I see regression tests passing, and I am supportive to merge.
@AreWeDreaming @smithsp @torrinba @hassec thoughts?

[AreWeDreaming]

I certainly don't use either of those with OMAS directly, but then I don't do a lot of database work overall.

[hassec]

Thanks for the ping @orso82, no objections from my side 👍

[smithsp]

Good by me. Good to know.