ipn/store/aws, cmd/tailscaled: add AWS SSM ipn.StateStore implementation

From https://github.com/tailscale/tailscale/pull/1919 with
edits by bradfitz@.

This change introduces a new storage provider for the state file. It
allows users to leverage AWS SSM parameter store natively within
tailscaled, like:

    $ tailscaled --state=arn:aws:ssm:eu-west-1:123456789:parameter/foo

Known limitations:
- it is not currently possible to specific a custom KMS key ID

RELNOTE=tailscaled on Linux supports using AWS SSM for state

Edits-By: Brad Fitzpatrick <bradfitz@tailscale.com>
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Signed-off-by: Maxime VISONNEAU <maxime.visonneau@gmail.com>
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