*: Bump version to v0.15.0 and add changelog entries

[mxinden]

Follow up to #53 and #56.

[dignifiedquire]

lgtm, but it is not actually a breaking change, is it?

[mxinden]

lgtm, but it is not actually a breaking change, is it?

In case a user exhaustively matches on Protocol, compilation will fail due to the new variant Protocol::Noise.

[dignifiedquire]

then maybe add non_exhaustive to the enzm for easier additions in the future?

[mxinden]

then maybe add non_exhaustive to the enzm for easier additions in the future?

I would argue that as a downstream user I would appreciate the breaking change in Protocol, i.e. I would appreciate to be notified of the change (additional variant) by the compiler. There are some downstream users who would want to make use of the new protocol, e.g. a Prometheus exporter which is tracking the observed protocols as a metric.