[secure-transport] reorder method definitions in cpp file (#11082)

This commit improves the organization of `secure_transport.cpp` by
grouping method definitions for `SecureSession` and `SecureTransport`
into separate sections.

The recent PR #11046 introduced `SecureSession`, separating the
session and transport functionality. To keep the `git diff` smaller
and easier to review, the methods were left in the same order as
previously defined. This resulted in methods of `SecureSession` and
`SecureTransport` being mixed and interleaved. This PR rearranges the
methods and defines separate sections for each class. This commit
does not contain any code logic changes.