Add Session.assume_role method

[benkehoe]

This is a followup on boto/botocore#2096. That was an overcomplicated implementation that used a custom CredentialProvider and credential chain, when botocore.Session already has a mechanism for overriding the credential chain with a Credentials object.

Issue boto/botocore#761 asks for first-class support for sts:AssumeRole, where there is a Session.assume_role() method that produces another session. This implements it by using the existing AssumeRoleCredentialFetcher and DeferredRefreshableCredentials classes, the latter of which is set as the _credentials field of the new botocore Session (like happens when you provide explicit credentials when setting up a session by calling set_credentials() on the botocore session; that method doesn't take a Credentials object, so this changes the field directly, but that could be fixed by a small change to botocore.Session). It sets the region of the assumed role session to the current value returned by Session.region_name.

I'm opening this PR for discussion as to whether it might be accepted, given that it's a substantially smaller change than proposed in the previous PR. As such, I have not yet implemented tests, nor given the method signature any thought (compare with the comprehensive interface provided by aws-assume-role-lib).

[kapilt]

typo

[nabebaye]

Bumping for visibility