Werk #15692: Add Sha256 signing for agents

The signature of baked agents uses SHA1 which is no longer state of the art. With this Werk we add SHA256 signatures. So if packages are signed now a SHA1 and a SHA256 signature is generated for that package. The agent updater can already validate these SHA256 signatures since version 2.2.0b9 but will also validate SHA1 signatures for compatibility reasons. Checkmk 2.3.0 won't generate SHA1 signatures anymore so make sure the agents are properly updated to version 2.2.0p8 or later. For the removal of SHA1 signatures in Checkmk 2.3.0 another Werk will be created and added here.

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