Configurable damage tick when blocking with shield (#10877)

A long standing bug in spigot and its derivatives was the fact that
players taking damage while blocking with a shield would not receive
invulnerability, while they do in vanilla.

This enabled the pvp technique of disabling a shield and immediately
attacking again to knock a player into the air.
While upstream fixed this and properly aligned itself with vanilla
damage logic (in this specific case) changing such long standing
behaviour has some downsides.

To allow players used to this specific bug to still use it, this patch
introduces a configuration option to re-introduce said bug.
As there is no easy way to *only* re-add this bug, the option is found
in the unsupported section as it may introduce other damage related
disparity from vanilla.

patches/server/Paper-config-files.patch

@@ -651,6 +651,8 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
 +        public boolean performUsernameValidation = true;
 +        @Comment("This setting controls if players should be able to create headless pistons.")
 +        public boolean allowHeadlessPistons = false;
++        @Comment("This setting controls if the vanilla damage tick should be skipped if damage was blocked via a shield.")
++        public boolean skipVanillaDamageTickWhenShieldBlocked = false;
 +        @Comment("This setting controls what compression format is used for region files.")
 +        public CompressionFormat compressionFormat = CompressionFormat.ZLIB;
 +