feat: make scheme configurable with default set to https

[shreemaan-abhishek]

Description

The default scheme of accessing the CLS was set to http which is a security shortcoming. We fix this issue by making the scheme configurable and set the default to https for better security.

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)