Add support for temporary changes to max hp

[mpirnat]

Confusingly, spells like Aid https://www.dndbeyond.com/spells/aid don't add temporary hit points but instead temporarily up the hit point max and add actual, non-temporary hit points. A specter's Life Drain attack can temporarily reduce the hit point max. We need to support this too (since both of these just happened in game!).

We should add a temp_max_hp attribute (or maybe max_hp_bonus?) to combatants.

If we choose to track the hp delta, then this attribute can default to 0, with a minimum of 0. The max_hp getter should return the sum of _max_hp and temp_max_hp. This will allow setting cur_hp above the usual _max_hp and instead up to the new limit.

If we choose instead to track the absolute override of max_hp, then this attribute should default to None. The max_hp getter should instead return the temp_max_hp value. This more easily accounts for reductions in max_hp so might be the way to go. Like the above approach, using the getter to do this allows the cur_hp setter to Just Work™ as it has.

The temp_max_hp should be emitted into the toml when saving player state, and should be reloaded when loading the party.

At the point where #71 is implemented, a long rest should optionally return the temp_max_hp to 0 (or None) -- perhaps with a prompt for any characters where there's an override set.

We can optionally add a temp_max_hp command to set the temp_max_hp on a combatant, or we can lean on the existing alter command which would already be able to do alter <combatant> temp_max_hp <value>.

[mpirnat]

Decisions made to make life easy:

1. Yeah, we're just setting an override of the max_hp. This hews closer to the game mechanics anyway.
2. I didn't add a command for setting/removing the max_hp_override, so for now this can be done using the alter command. It can be removed by doing alter <combatant> max_hp_override None.
3. If the value given to the alter command is "None", we turn that into a Python None rather than the string. Sorry to anyone who wanted to do alter <combatant> name None!