Add ssh-agent support

Today the log's private key is stored and accessed in plaintext.

Adding SSH agent support would give the log operator a large degree of flexibility on how to manage access to their private key. For example, it would be possible to configure each log node with a "small HSM on USB".

@nisse already has a poc on how to perform a signing operation in Go using ssh agent, see https://git.glasklar.is/nisse/sigsum-tools/-/tree/main/agent