refactor: use setmetatable to set hidden variables without effecting serialisation

[Revolyssup]

Description

Users hope to directly use the decoded body in the template of body-transformer, modify its fields, and then encode it again. Previously, we set several hidden variables in the _out variable, including function-type variables, which caused issues with direct encoding.

Fixes # (issue)

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)