feat: introduce signer

[frisitano]

closes: #61

[greged93]

Just a general comment: the Signer runs as a separate task, which moves away from the RNM centric view. I'm fine with it but why go in this direction? Do you think we should move more components to separate tasks (related to #57)?

[frisitano]

@greged93 The reason for moving the Signer into a separate task is that signing is order-independent. I.e. we don't care if we perform a block import, index, or build a block before we sign. As such, we can delegate polling to the tokio executor. We only need to keep components in the same task if the ordering of component execution has a tangible impact on the system as a whole.

[greged93]

lgtm!