Move the Logappender module to external

[duhenglucky]

FEATURE REQUEST

1. Please describe the feature you are requesting.
   Based on the discussion and vote result of separate logappender repo, move logappender the to rocketmq-external repo.

2. Provide any additional detail on your proposed use case for this feature.

3. Indicate the importance of this issue to you (blocker, must-have, should-have, nice-to-have). Are you currently using any workarounds to address this issue?

4. If there are some sub-tasks using -[] for each subtask and create a corresponding issue to map to the sub task:

• sub-task1-issue-number: sub-task1 description here,
• sub-task2-issue-number: sub-task2 description here,
• ...