MDEV-28911: Add --init-command option to mysqldump

[HNOONa-0]

• The Jira issue number for this PR is: MDEV-28911

Description

A quick note on the implementation: I set it up so the command runs before mariadb-dump initializes its own default session settings. I originally thought it made more sense to put it after (so users could override the tool's internal defaults), but looking at the rest of the codebase, doing it before seems to be the established pattern. I decided to stick with the existing convention to keep things consistent. I also added a small MTR test file to verify the flag works as expected. Do you agree with this order of execution?

Right now, only the last init-command option will run. Is it desirable to allow users to specify multiple init commands?

Release Notes

Add --init-command option to mysqldump

How can this PR be tested?

Basing the PR against the correct MariaDB version

• 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.

PR quality check

• 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.

[gkodinov]

Thank you for your contribution! This is a preliminary review.

Please consider extending the MDEV text with a full functional description of the new option so it could be documented well.

Also, please make sure the text of your commit message complies with CODING_STANDARDS.md.

Some answers to your questions below.

[HNOONa-0]

failed tests work fine on my machine

[gkodinov]

LGTM. Thank you for taking the extra effort to implement the common headers!

Stand by for the final review.

[HNOONa-0]

Hello @gkodinov, I've implemented some of the modifications you suggested. For now, I've only applied them to a single client (mysql.cc) as a proof of concept. If you're happy with this selection of variables, I will apply them to other client files and maybe add a couple of tests too.