feat: Add pending spinner icon for pending messages (draft)

[liamcho]

Ticket

https://sendbird.atlassian.net/browse/AC-2609

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[github-actions[bot]]

Size Change: +319 B (+0.05%)

Total Size: 616 kB

Filename	Size	Change
./dist/__bundle-91b5ecea-********.js	892 B	-1 B (-0.11%)
./dist/index-********.js	313 kB	+150 B (+0.05%)
./dist/index.es.js	171 B	-1 B (-0.58%)
./dist/index.umd.js	290 kB	+171 B (+0.06%)

ℹ️ View Unchanged

| Filename | Size | | :--- | :---: | | `./dist/__bundle-46d64517-********.js` | 4.01 kB | | `./dist/__bundle-7c3d40f4-********.js` | 21 B | | `./dist/style.css` | 7.93 kB |

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[bang9]

It seems that we frequently need to ask for confirmations. Please check the following list before requesting a review:

• Ensure that the ticket requirements have been clearly implemented.
• Verify that all CI checks have passed.
• Conduct a self-review and further check for any areas of improvement or discrepancies with the requirements.

[bang9]

@liamcho 이거는 계속 draft 로 남아있는데 티켓은 in review 상태인거같네요, 작업을 진행하던가 티켓 상태를 업뎃하면 좋을 것 같습니다.

[liamcho]

@bang9 폼 작업 -> 템플릿 작업 -> 이후 재개 할 예정이라 백로그로 변경했습니다

[liamcho]

duplicate of https://github.com/sendbird/chat-ai-widget/pull/363