feat: display a warning message when the data plane has write access to etcd

[LiteSun]

Description

In short, as long as APISIX is deployed in decoupled mode with the role set to data_plane, and it can successfully write to etcd, a warning message will be displayed.

A warning message will be displayed under the following conditions:

1. Etcd allows anonymous access, enabling the data plane to write to etcd.
2. The configured etcd user has write permissions.

No warning will be shown in the following case:

1. The configured etcd user has read-only permissions.

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)

[LiteSun]

After discussions with @moonming and @nic-6443, we found that checking etcd write permissions during the APISIX startup phase is not very accurate, especially under different etcd cluster permission configurations. A new PR will be submitted to display a warning message when a write operation to etcd is attempted.