ResourcePacksInfoPacket: Added missing setter

[ItzxDwi]

Resolves pmmp/PocketMine-MP#6739, pmmp/PocketMine-MP#6740

[ItzxDwi]

someday 🙏

[dktapps]

As I mentioned in pmmp/PocketMine-MP#6739, we don't actually need this API since you can just create a new packet and use DataPacketSendEvent::setPackets(). There's some uncertainty within the team about whether to allow mutating this, since we've been moving towards immutable packets for some time now.

[dktapps]

Related to #330

[JaxkDev]

Just bumping this as a plugins come in, with its whole purpose of reflecting forceDisableVibrantVisuals to change it to true.
(Yeah they could rebuild it and set the packets in the event etc)

But after checking with dries, not sure why this field was private to begin with? especially if this PR adds a setter, seems like it should be changed to public. (leave the getter for BC if you really want)

Or was this intentionally private as all? fields now get added as private with getter?

[dktapps]

See #330