Fix EncryptedPaginator to successfully decrypt

[mattsb42-aws]

Issue #, if available:
#118

Description of changes:

• Added integration tests that test the scan paginator with KMS crypto materials providers.
• Single-sourced logic for decrypting a list of items, and updated both EncryptedPaginator and decrypt_multi_get to use it.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

Check any applicable:

• Were any files moved? Moving files changes their URL, which breaks all hyperlinks to the files.

[ragona]

LGTM.

[SalusaSecondus]

LGTM, with the following two comments:

1. I'm not a python dev.
2. Please add a specific regression test to the non-integration tests to catch this specific bug (or more generally if possible)

[mattsb42-aws]

LGTM, with the following two comments:

2. Please add a specific regression test to the non-integration tests to catch this specific bug (or more generally if possible)

Added.
I also verified that these test changes break the EncryptedPaginator tests as expected if the other changes in this PR are not applied.