Propagates SECURITY LABEL ON ROLE stmt (#7304)

[emelsimsek]

We propagate SECURITY LABEL [for provider] ON ROLE rolename IS labelname to the worker nodes.
We also make sure to run the relevant SecLabelStmt commands on a newly added node by looking at roles found in pg_shseclabel.

See official docs for explanation on how this command works: https://www.postgresql.org/docs/current/sql-security-label.html This command stores the role label in the pg_shseclabel catalog table.

This commit also fixes the regex string in
check_gucs_are_alphabetically_sorted.sh script such that it escapes the dot. Previously it was looking for all strings starting with "citus" instead of "citus." as it should.

To test this feature, I currently make use of a special GUC to control label provider registration in PG_init when creating the Citus extension.

(cherry picked from commit 0d1f188)

DESCRIPTION: PR description that will go into the change log, up to 78 characters

[codecov]

Codecov Report

Attention: Patch coverage is 89.41176% with 9 lines in your changes missing coverage. Please review.

Project coverage is 89.65%. Comparing base (15ecc37) to head (8cb73d9).
Report is 1 commits behind head on release-12.1.

Additional details and impacted files

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##           release-12.1    #7735      +/-   ##
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+ Coverage         89.64%   89.65%   +0.01%     
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  Branches           7435     7446      +11     
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+ Hits              53407    53488      +81     
+ Misses             4036     4034       -2     
- Partials           2132     2135       +3