feat: support configuring the buffer size of the access log

[qihaiyan]

Description

access log's buffer size can be configed.

Access log buffering can significantly reduce disk I/O and improve server performance. Enabling access log buffering is relatively simple in NGINX. To do this, use the following directive in your NGINX configuration:

access_log /var/log/nginx/access.log main buffer=32k;
This directive tells NGINX to store the access log entries in a buffer of size 32k before writing them to the log file. Increasing the buffer size can minimize the number of write operations to the disk, resulting in improved performance.

Fixes #10219

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

[shreemaan-abhishek]

Please also describe why this is needed.

[qihaiyan]

Please also describe why this is needed.

I enriched the description.