PS-10321 fix: binlog-server fails to read the new events after upgrading server from 8.0 to 8.4

[percona-ysorokin]

https://perconadev.atlassian.net/browse/PS-10321

Fixed an issue with Binlog Server utility not being able to process FORMAT_DESCRIPTION events generated by previous versions of MySQL Server (e.g. 8.0 FDEs coming from 8.4 server). This may happen when user performs an in-place upgrade of his data directory and has old binlog files with 8.0 FDEs along with new binlog files with 8.4 FDEs, which are 1 byte larger.

Reworked the way how 'binsrv::events::event_view' class determines the boundaries of event sections (common_header / post_header / body / footer) for FORMAT_DESCRIPTION event. Because of this, it is now possible to parse FDEs generated on different versions of the MySQL Server (which are different in size).

Simplified 'binsrv::events::event:' class byte span constructor - it no longer requires 'reader_context' as a parameter.

Reworked FORMAT_DESCRIPTION post header class constructor - it no longer expects a data span of pre-defined length. Instead it checks the length dynamically based on the 'server_version' extracted on the way.

In 'binsrv::events::reader_context' class we now differentiate between encoded server version specified during object construction and coming from the MySQL connection object ('get_connection_encoded_server_version()' method) and encoded server version encountered in the last processed FORMAT_DESCRIPTION event ('get_current_encoded_server_version()' method).

Added 'binlog_streaming.data_directory_8_0_to_8_4_upgrade' MTR test case that checks if the Binlog Server utility can properly download events from an instance of MySQL Server after 8.0 -> 8.4 upgrade. The following script was used to generate original 8.0 binlog files: CREATE TABLE t1(id SERIAL, PRIMARY KEY(id));
INSERT INTO t1 VALUES();
INSERT INTO t1 VALUES();

START TRANSACTION;
INSERT INTO t1 VALUES();
INSERT INTO t1 VALUES();
INSERT INTO t1 VALUES();
INSERT INTO t1 VALUES();
COMMIT;

FLUSH BINARY LOGS;

UPDATE t1 SET id = id + 100 WHERE id <= 3;
DELETE FROM t1 WHERE id < 100;
The test has 2 combinations: 'position' and 'gtid'. For each of them an individual data directory archive was created.