Further warning label in Rewards log

[fmarier]

As a follow-up to brave/brave-core#6187, let's add a similar notice inside the actual log file.

That way the warning label will "travel" with the log wherever it might go, and a user who downloaded the log a while back and later uploads it somewhere will get another chance to see the warning (if they peek inside the file).

It might also be worth naming the file brave_rewards_sensitive_log.txt or something similar.

[mkarolin]

Test plan:

1. Start Brave and navigate to brave://rewards-internals
2. Click on the Logs tab
3. Click on the Download full log button
4. Observe the Save As dialog: the default file name should be brave_rewards_sensitive_log.txt
5. Click Save
6. Open the saved file
7. Verify that the first line of the file is: WARNING: This log file may contain sensitive data. Be careful who you share it with.

[stephendonner]

Verification PASSED using

Brave	1.54.82 Chromium: 115.0.5790.40 (Official Build) beta (64-bit)
Revision	071c9ddea889c3c7887daf4eac13fed72d4fff62-refs/branch-heads/5790@{#979}
OS	Windows 10 Version 22H2 (Build 19045.3155)

Steps:

1. installed 1.54.82
2. launched Brave
3. skipped through onboarding
4. clicked on Start using Rewards on the new-tab page
5. chose United States from the country-picker dropdown
6. opened brave://rewards-internals/
7. clicked on the Logs tab
8. clicked on the Download full log button
9. clicked Save
10. opened the saved log file
11. checked the first line of the file

Confirmed the line reads WARNING: This log file may contain sensitive data. Be careful who you share it with.

example	example	example	example