Improve Kinesis event recort type

[calavera]

Issue #, if available:

Fixes #858

Description of changes:

Make the encryption type an enum. Make sequence number and partition key non-optional.

By submitting this pull request

• [x] I confirm that my contribution is made under the terms of the Apache 2.0 license.
• [x] I confirm that I've made a best effort attempt to update all relevant documentation.