let icons be settable by loading from server endpoint

[dceejay]

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Proposed changes

This PR lets editor node icons be createable (eg from a library) rather than purely static. They are only created at load time (so not fully dynamic... it's not a dashboard after all :-) .

They can then be set in the html file by creating an admin endpoint eg (using th feather-icons package

  RED.httpAdmin.get('/simple/icon/:id', (req, res) => {
    const sym = req.params.id.split('.')[0];
    res.writeHead(200, { "Content-Type": "image/svg+xml" });
    res.write(feather.icons[sym].toSvg({width:24, height:24, viewBox:'-4 -4 32 32'}));
    res.end();
});

and then called by something like

icon: function() {return '/simple/icon/' + this.name + '.svg' },

In this PR the server side does expect the node.type (in the lines above = example) and to end with .svg - but this can be discussed changed in the PR... eg it could also be extended to allow .png - but creating dynamic png would be harder...

NOTE: it does mean that the get function above must return an SVG of suitable size - ie about 35-40px wide.

Checklist

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the forum/slack team.
• I have run npm run test to verify the unit tests pass
• I have added suitable unit tests to cover the new/changed functionality

[dceejay]

I'd still like to get this into 4.0 if possible,

[knolleary]

I'm okay with the principle... just want to think about the implementation a bit and how this sits with existing ways a node can provide icons and our general API structure.

We already provide a way for a node to expose additional resources to the editor (https://nodered.org/docs/creating-nodes/resources) which get exposed under /resources/<module>/<file>

We also track what static icons a node provides and they get exposed under /icons/<module>/<file>

I'd like to find an api design that feels part of this structure - just not sure what that is yet. Will have a poke at how the existing routes are managed.