[announce] Allow raw messages to be sent to Mattermost

[aalmiray]

Discussed in #1079

^{Originally posted by cyrillesondag December 1, 2022}
hi,

In annoncers (at least in Mattermost ) wouldn't it be more richfull to templatize the raw post body ?

So like this you could have either a "simple text" message with "text" property, or a more elaborate message with the "template"

For exemple some thing like this :

Object message;
if (isNotBlank(mattermost.getMessage())) {
    message = Message.of(mattermost.getResolvedMessage(context));
} else {
    Map<String, Object> props = new LinkedHashMap<>();
    props.put(Constants.KEY_CHANGELOG, MustacheUtils.passThrough(context.getChangelog()));
    context.getModel().getRelease().getReleaser().fillProps(props, context.getModel());
    message = mattermost.getResolvedMessageTemplate(context, props);  // raw json message body
}
```</div>

[aalmiray]

Released in v1.4.0 -> https://github.com/jreleaser/jreleaser/releases/tag/v1.4.0