Previously we had a "getProvider" function type used to implement plugin
fetching. Here we replace that with an interface type, initially with
just a "Get" function.
For now this just simplifies the interface by allowing the target
directory and protocol version to be members of the struct rather than
passed as arguments.
A later change will extend this interface to also include a method to
purge unused plugins, so that upgrading frequently doesn't leave behind
a trail of unused executable files.
We can filter the allowed versions and sort them before checking the
protocol version, that way we can just return the first one found
reducing network requests.
Extend the test reslease server to return the protocol version header
and a dummy zip file for the provider.
Test filtering the plugins by plugin protocol version and add a full
GetProvder test.
Martin Atkins