feat(Core): make BGs and Arena prep time configurable

[NoxMax]

Apologies for reposting the PR. I accidentally closed the original -_-

Changes Proposed:

PR makes BGs and Arena prep time configurable in worldserver.conf. Keeping the default values of 120s/60s for BGs and Arena respectively. Now you don't have to wait if the default times are too long for you.
Making the timing configurable was simple, but making the timing announcements accurate is what required most of the work. Previous code logic had announcements triggered based on event flags rather than actual time values (full/half/quarter/none) The new code makes _ProcessJoin in Battleground.cpp actually aware how much time is left till match begins.
This PR proposes changes to:

• Core (units, players, creatures, game systems).
• Scripts (bosses, spell scripts, creature scripts).
• Database (SAI, creatures, etc).

Issues Addressed:

None on GH, but requested feature on Discord.

SOURCE:

N/A Default behavior has not been changed.

Tests Performed:

Compiled correctly on Debian.
Changes behaved as expected in-game.
This PR has been:

• Tested in-game by the author.
• Tested in-game by other community members/someone else other than the author/has been live on production servers.
• This pull request requires further testing and may have edge cases to be tested.

How to Test the Changes:

1. At the default values of 120s/60s for BGs and Arena, there should be no observed change in game.
2. Change the prep time of BGs and Arena to something other than 120s/60s and restart the server. The prep time should be whatever you changed the value to.
3. The announcement messages should be accurate. If it says 30 seconds till start, match must start in 30 seconds.
4. BGs have announcements at 120/60/30/0 seconds. Arena has announcements at 60/30/15/0 seconds. If you set a prep time that is less than one of the announcement times, you should not see an announcement for a time that is greater than the configured perp time. Example: If you set the prep time to 45 seconds for BGs, you should only get the 30/0 second announcements, and at their correct time.

Please advise after testing if anything has been missed.

Known Issues and TODO List:

None for now.

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