Add '/assign' command to Radius bot (radius-project#6550)

# Description

Adds support to assign issues via Radius bot with the `/assign` command.
Any contributor can issue the command and assign an unassigned issue to
themself.

## Type of change

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- This pull request is a minor refactor, code cleanup, test improvement,
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Fixes: radius-project#6549

## Auto-generated summary

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copilot:all
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### <samp>🤖 Generated by Copilot at 10558f5</samp>

### Summary
🤖🙋🎯

<!--
1. 🤖 - This emoji represents the radius bot itself, as well as the
automation and functionality that it provides to the users.
2. 🙋 - This emoji represents the self-assignment feature, as well as the
user's intention and initiative to take on an issue.
3. 🎯 - This emoji represents the issue that is being assigned, as well
as the focus and goal that the user has for working on it.
-->
Add `/assign` command to `radius-bot.js` to enable self-assignment of
issues.

> _Sing, O Muse, of the mighty radius bot_
> _That heeds the users' wishes with `/assign`_
> _And grants them issues to resolve with skill and thought_
> _Unless by pull request or prior claim they're bound._

### Walkthrough
* Add a new command `/assign` to the issue comment handler in
`radius-bot.js`
([link](https://github.com/radius-project/radius/pull/6550/files?diff=unified&w=0#diff-b94c0e124b5b65fb6fa8785928c5331547c124a96ecc225f93df9445a29a5109R44-R46),
[link](https://github.com/radius-project/radius/pull/6550/files?diff=unified&w=0#diff-b94c0e124b5b65fb6fa8785928c5331547c124a96ecc225f93df9445a29a5109R57-R80))

---------

Signed-off-by: Shubham Sharma <[email protected]>
Signed-off-by: willdavsmith <[email protected]>

.github/scripts/radius-bot.js

@@ -41,6 +41,9 @@ async function handleIssueCommentCreate({ github, context }) {
     const command = commandParts.shift();
 
     switch (command) {
+        case '/assign':
+            await cmdAssign(github, issue, isFromPulls, username);
+            break;
         case '/ok-to-test':
             await cmdOkToTest(github, issue, isFromPulls, username);
             break;
@@ -50,6 +53,30 @@ async function handleIssueCommentCreate({ github, context }) {
     }
 }
 
+/**
+ * Assign issue to the user who commented.
+ * @param {*} github GitHub object reference
+ * @param {*} issue GitHub issue object
+ * @param {*} isFromPulls is the workflow triggered by a pull request?
+ * @param {*} username is the user who trigger the command
+ */
+async function cmdAssign(github, issue, isFromPulls, username) {
+    if (isFromPulls) {
+        console.log('[cmdAssign] pull requests not supported, skipping command execution.');
+        return;
+    } else if (issue.assignees && issue.assignees.length !== 0) {
+        console.log('[cmdAssign] issue already has assignees, skipping command execution.');
+        return;
+    }
+
+    await github.rest.issues.addAssignees({
+        owner: issue.owner,
+        repo: issue.repo,
+        issue_number: issue.number,
+        assignees: [username],
+    });
+}
+
 /**
  * Trigger e2e test for the pull request.
  * @param {*} github GitHub object reference