* MM-12193: remove auto configuration unmarshalling
Since plugin hook events are called concurrently, there's no way for the plugin framework to coordinate safe access to the automatically unmarshalled configuration fields. Remove this functionality, and update documentation to illustrate a safe way to do this.
* better Fprint example
* fix unit tests
* log when OnConfigurationChange fails through OnActivate
* clarify lifecycle when OnConfigurationChange returns an error
* call SetAPI even if OnConfigurationChange not implemented
* clean up plugins GoDoc:
- eliminate plugin.NewBlankContext() as unnecessary
- export ValidIdRegex as a string vs. the less readable var
- add/update various documentation strings
- hide everything by default, except where used by client plugins or the mattermost-server. The exception to this rule are the `*(Args|Returns)` structs which must be public for go-plugin, but are now prefixed with `Z_` with a warning not to use.
- include a top-level example to get plugin authors started
This is not a breaking change for existing plugins compiled against
plugins-v2.
* remove commented out ServeHTTPResponseWriter
* update examples to match developer docs
* add missing plugin/doc.go license header
