Fix output for binary keys in -verbose mode.

[burnison]

Description

When migrating a table with a binary key in -verbose mode, different sequences of bytes may prevent execution output from being logged, or correctly logged, to the console.

This happens because the MigrationRange*Values are printed to the screen without any type of encoding. One particularly problematic sequence is 27/0x1b, Escape, which causes the terminal to stop logging for the duration of the migration:

2025-05-23 11:53:27 INFO Listening on unix socket file: /tmp/gh-ost.test.binfoo.sock
2025-05-23 11:53:27 INFO Intercepted changelog state ReadMigrationRangeValues
2025-05-23 11:53:27 INFO Handled changelog state ReadMigrationRangeValues
2025-05-23 11:53:27 INFO Migration min values: [

This commit changes the to-string rendering for binary keys, rendering the values as a hex string rather than an unescaped series of bytes:

2025-05-23 11:53:58 INFO Listening on unix socket file: /tmp/gh-ost.test.binfoo.sock
2025-05-23 11:53:58 INFO Intercepted changelog state ReadMigrationRangeValues
2025-05-23 11:53:58 INFO Handled changelog state ReadMigrationRangeValues
2025-05-23 11:53:58 INFO Migration min values: [b51b0000000000000000000000000000]
2025-05-23 11:53:58 INFO Migration max values: [b51b0000000000000000000000000000]
2025-05-23 11:53:58 INFO Waiting for first throttle metrics to be collected
...

While this build passes and has the expected outcome locally, I'm not positive that this change to StringColumn is used only for logging/debugging purposes. This is something I hope you can verify.

In case this PR introduced Go code changes:

• contributed code is using same conventions as original code
• script/cibuild returns with no formatting errors, build errors or unit test errors.