MDEV-38601 require FEDERATED ADMIN for SHOW CREATE SERVER

[MooSayed1]

The Jira issue number for this PR is: MDEV-38601

Description

SHOW CREATE SERVER did not require any privilege — any authenticated user
could run it and see full server connection details including passwords stored
in mysql.servers.

-- Any user could run this without FEDERATED ADMIN:
SHOW CREATE SERVER srv;
-- Returns: CREATE SERVER `srv` ... OPTIONS (USER 'remote_user', PASSWORD 'secret', ...)

CREATE SERVER, ALTER SERVER, and DROP SERVER all correctly require
FEDERATED ADMIN privilege, but SHOW CREATE SERVER (introduced in MDEV-15696,
MariaDB 11.7) was missing the equivalent check.

The root cause is in sql/sql_parse.cc: the SQLCOM_SHOW_CREATE_SERVER case
had no check_global_access() call before dispatching to
mysql_show_create_server().

The fix: add check_global_access(thd, PRIV_STMT_CREATE_SERVER) at the top
of the SQLCOM_SHOW_CREATE_SERVER case, matching the identical pattern used
by SQLCOM_CREATE_SERVER, SQLCOM_ALTER_SERVER, and SQLCOM_DROP_SERVER.

Release Notes

SHOW CREATE SERVER now requires the FEDERATED ADMIN privilege.
Users without this privilege receive ERROR 42000: Access denied; you need (at least one of) the FEDERATED ADMIN privilege(s).

How can this PR be tested?

./mtr main.grant_server

The test creates a server as root, then verifies that an unprivileged user
receives ER_SPECIFIC_ACCESS_DENIED_ERROR on SHOW CREATE SERVER, and that
the same user succeeds after being granted FEDERATED ADMIN.

Basing the PR against the correct MariaDB version

This is a bug fix. SHOW CREATE SERVER was introduced in 11.7, and the
earliest affected maintained branch is 11.8, so this PR targets upstream/11.8.

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.

In addition to the below, please make sure your commit has a commit message complying with CODING_STANDARDS.md.

[MooSayed1]

@gkodinov @mariadb-YuchenPei Thank you for the review and srry for the delay was focusing at proposal and other stuff.

[mariadb-YuchenPei]

Looks good now. Passing to @vuvova for a final review.

[gkodinov]

Please add a commit message to your commit!

[MooSayed1]

@gkodinov Done. Thank you all for your effort in reviewing and guiding.

[gkodinov]

LGTM. Thank you for your work on this!