MDEV-10963 follow-up: MDEV-28768 mysqlbinlog can produce events larger than max_allowed_packet

[ParadoxV5]

copy_cache_to_file_wrapped, MDEV-10963’s original implementation, was not flashback-compatible.
copy_cache_to_string_wrapped, a version for flashback, replaced it in the next MariaDB version, and soon they merge conflicted. The resolution was to keep both, complete with all the duplicate code (that differ only at how they output their chars).

This commit

1. replaces those differing outputters with reüsable C++ lambdas
2. merges the common portion to a single function.

---

• The Jira issue number for this PR is: MDEV-28768

Description

TODO: An amazing description should answer some questions like:

1. What problem is the patch trying to solve?
2. If some output changed that is not visible in a test case, what was it looking like before the change and how it's looking with this patch applied?
3. Do you think this patch might introduce side-effects in other parts of the server?

Release Notes

TODO: What should the release notes say about this change?
Include any changed system variables, status variables or behaviour. Optionally list any https://mariadb.com/kb/ pages that need changing.

How can this PR be tested?

TODO: modify the automated test suite to verify that the PR causes MariaDB to behave as intended.
Consult the documentation on "Writing good test cases".

Behavior & Performance: If the changes are not amenable to automated testing, please explain why not and carefully describe how to test manually.

PR quality check

• This is a new feature or a refactoring, and the PR is based against the main branch.
• This is a bug fix, and the PR is based against the earliest maintained branch in which the bug can be reproduced.
• I checked the CODING_STANDARDS.md file and my PR conforms to this where appropriate.
• For any trivial modifications to the PR, I am ok with the reviewer making the changes themselves.