feat: avoid overriding customized X-Forwarded-For header

[wangfeng22]

Description

Fixes #8195

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)

[tokers]

Need test cases to cover the changes.

[wangfeng22]

Need test cases to cover the changes.

@tokers It's similar to #4260, and I don't know how to write test cases.

[tzssangglass]

@tokers It's similar to #4260, and I don't know how to write test cases.

read more: https://github.com/apache/apisix/blob/master/docs/en/latest/plugin-develop.md#write-test-case

[wangfeng22]

@tokers It's similar to #4260, and I don't know how to write test cases.

read more: https://github.com/apache/apisix/blob/master/docs/en/latest/plugin-develop.md#write-test-case

@tzssangglass The test tool does not meet the testing needs, because the realip_remote_addr in the returned header is dynamic.

[tzssangglass]

@tzssangglass The test tool does not meet the testing needs, because the realip_remote_addr in the returned header is dynamic.

we can change:

apisix/apisix/cli/ngx_tpl.lua

Lines 677 to 679 in 024e0ef

	if ($http_x_forwarded_for != "") {
	set $var_x_forwarded_for "${http_x_forwarded_for}, ${realip_remote_addr}";
	}

and

apisix/t/APISIX.pm

Lines 771 to 773 in 024e0ef

	if (\$http_x_forwarded_for != "") {
	set \$var_x_forwarded_for "\${http_x_forwarded_for}, \${realip_remote_addr}";
	}

[tzssangglass]

you can learn from: https://github.com/apache/apisix/pull/8200/files

[wangfeng22]

you can learn from: https://github.com/apache/apisix/pull/8200/files

@tzssangglass Thank you very much. I have added some test cases.