Add support for assumed_role and STS

[ShimiTaNaka]

Add support to get assumed_role credentials and add option to set default STS TTL and max STS TTL

[ShimiTaNaka]

I have ran tests and acceptance tests against Vault 1.1.0 without AWS keys, so some of the tests didn't pass.
Would be happy to know if there are scenarios in which the tests fail.

[tyrannosaurus-becks]

Wow, @ShimiTaNaka , thanks for adding all this, and with such great test coverage.

It's conflicting with master now because of how I merged #407 which adds support for role_arns. Would you be willing to merge in master and resolve the conflicts? After that I'll be able to run the tests with an AWS access key and secret, and if they don't pass I'll post the output; but if they do then this should be good to go.

[ShimiTaNaka]

@tyrannosaurus-becks Looking Again on my changes, since I took another PR that wasn't maintained, I didn't notice I'm also adding the role_arns
Since there are few relatively important differences, I suggest having another pair of eyes
(For example, now in master, policy is marked as conflicting with role_arns, however, I do not see any evidence for it in the documentation, contrary, it seems that you can add both and have the policy as extra filter in addition to the roles
This obviously also affects the rest of the code.
@oxlay Would love your input as well since the conflicting PR is yours if possible :)
Thanks

[ShimiTaNaka]

@tyrannosaurus-becks any updates?

[tyrannosaurus-becks]

Happy to take a look at this once master is merged in!

[tyrannosaurus-becks]

Closing this for now due to inactivity. If you end up wanting to reopen this, feel free to do so when you're ready. Thank you for your work!