ipn/store/awsstore: allow providing a KMS key

Implements a KMS input for AWS parameter to support encrypting Tailscale
state

Fixes #14765

Change-Id: I39c0fae4bfd60a9aec17c5ea6a61d0b57143d4ba
Co-authored-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Signed-off-by: Lee Briggs <lee@leebriggs.co.uk>
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