Set version string in initial server response

[nicktobey]

MySQL servers return a version string in the initial connection response. Some clients read this and expect certain versions of MySQL or MariaDB. We return 8.0.31 by default, since that corresponds with the MySQL version we advertise feature parity with.

Currently, we use the @@version system variable as the source of truth for the version string, and this can be configued via a server config file. But we extract the config for the listener before we initialize system variables. Previously this meant that the version string on connection defaulted to the value specified in our vitess dependency, which was hardcoded to 8.0.31.

This PR changes the behavior to extract a string from the config if it exists.

We need to use a client integration test for this, since the Go SQL driver ignores the version in the connection info and does not provide a way to view it.

[coffeegoddd]

@nicktobey DOLT

read_tests	from_latency	to_latency	percent_change
covering_index_scan	2.3	2.3	0.0
groupby_scan	139.85	134.9	-3.54
index_join	2.0	1.89	-5.5
index_join_scan	1.34	1.34	0.0
index_scan	215.44	207.82	-3.54
oltp_point_select	0.26	0.26	0.0
oltp_read_only	5.09	5.09	0.0
select_random_points	0.55	0.55	0.0
select_random_ranges	0.64	0.64	0.0
table_scan	196.89	200.47	1.82
types_table_scan	458.96	458.96	0.0

write_tests	from_latency	to_latency	percent_change
oltp_delete_insert	6.21	6.21	0.0
oltp_insert	3.13	3.13	0.0
oltp_read_write	11.24	11.24	0.0
oltp_update_index	3.3	3.3	0.0
oltp_update_non_index	3.02	3.02	0.0
oltp_write_only	6.32	6.21	-1.74
types_delete_insert	6.79	6.79	0.0

[coffeegoddd]

@nicktobey DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
2315012	ok	5937471

version	total_tests
2315012	5937471

correctness_percentage
100.0

[coffeegoddd]

@nicktobey DOLT

test_name	from_latency_p95	to_latency_p95	percent_change
tpcc-scale-factor-1	44.98	45.79	1.8

test_name	from_server_name	from_server_version	from_tps	to_server_name	to_server_version	to_tps	percent_change
tpcc-scale-factor-1	dolt	1bf5332	52.57	dolt	2315012	52.6	0.06

[macneale4]

LGTM