Handle plugin name in plugins.getConfig

[GogoVega]

Types of changes

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

Proposed changes

If I do a GET request to /plugins/my-plugin-module/my-plugin, the response should be the configuration of the my-plugin plugin and not each configuration contained in my-plugin-module.

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

[GogoVega]

I split the actual route into two:

• the first to get the module info. This amounts to calling the same route for nodes, as there's no distinction at this level.
• the second to get the plugin configuration. The Accept header has been adapted to handle application/json and text/plain results.

[GogoVega]

Now #5277 is merged, when a user installs a module containing plugins from the Palette Manager, the editor will fetch for the configuration of each plugin.

To do this, the editor receives an array with plugin information and will then make a request per plugin to retrieve its configuration (its HTML content).

node-red/packages/node_modules/@node-red/editor-client/src/js/red.js

Lines 556 to 566 in b939305

	$.ajax({
	headers: {
	"Accept":"text/html",
	"Accept-Language": lang
	},
	cache: false,
	url: 'plugins/'+id,
	success: function(data) {
	appendPluginConfig(data);
	}
	});

The problem is that the request returns the contents of all plugins in that module instead of the one requested. I mean the request contains the plugin ID but the function is called with the module name.

node-red/packages/node_modules/@node-red/editor-api/lib/admin/plugins.js

Lines 44 to 50 in b939305

	getConfig: function(req, res) {
	let opts = {
	user: req.user,
	module: req.params[0],
	req: apiUtils.getRequestLogObject(req)
	}

node-red/packages/node_modules/@node-red/registry/lib/plugins.js

Lines 86 to 99 in b939305

	function generateModulePluginConfig(module) {
	let result = '';
	const plugins = module.plugins
	for (let plugin in plugins) {
	if (plugins.hasOwnProperty(plugin)) {
	let config = plugins[plugin];
	if (config.enabled && !config.err && config.config) {
	result += "\n<!-- --- [red-plugin:"+config.id+"] --- -->\n";
	result += config.config;
	}
	}
	}
	return result;
	}

As a result, if the array contains two plugins, the onadd method will be called twice for each plugin.

To solve this problem I propose to get closer to the functioning of nodes; getConfig will return the contents of the specified plugin as getSet does for nodes.