parseComBinlogDumpGTID: GTID payload is always 5.6 flavor

[shlomi-noach]

Description

Quick followup/fix to #17580. #17580 wasn't well tested, and although nothing in Vitess is actually calling ComBinlogDumpGTID with a GTID payload -- such calls fail. This is because the function attempts to run:

		position, err = replication.DecodePosition(gtid)

However, the encoded GTID does not have a MySQL56/ prefix. Rightly so, because that's not in the MySQL protocol. The command ComBinlogDumpGTID is MySQL-specific (ie not MariaDB or any other flavor) and thus the GTID payload is necessarily a MySQL56 flavor.

In addition, the GTID is written in ComBinlogDumpGTID as a SID block. This PR now also parses it back from a SID block form.

This PR fixes the issue and adds appropriate testing.

Related Issue(s)

#17579

Checklist

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[mattlord]

Nit, but no reason not to use decimal here (can then remove the comment).

[shlomi-noach]

done.

[codecov]

Codecov Report

Attention: Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 67.66%. Comparing base (aaee12a) to head (d574d77).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
go/mysql/binlog_dump.go	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17605      +/-   ##
==========================================
- Coverage   67.68%   67.66%   -0.03%     
==========================================
  Files        1586     1586              
  Lines      255170   255407     +237     
==========================================
+ Hits       172717   172825     +108     
- Misses      82453    82582     +129     

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