Prevent copy-from from discarding armour values

[dseguin]

Summary

None

Purpose of change

Fixes #53130.
There are really 2 issues:

1. The "material_thickness", "environmental_protection" and "environmental_protection_with_filter" fields are discarded from the parent object when using copy-from.
2. Attempting to use "proportional" or "relative" on the fields above causes an exception.

Describe the solution

1. Store the values in the islot_armor struct so that a copied instance can reuse the parent's values.
2. Bypass cata::optional when reading the values from JSON. The "proportional" and "relative" fields require the value type to support the *= and += operators, respectively.

Describe alternatives you've considered

#54681 used a workaround, but doesn't solve the underlying problem.

Testing

Added test case to verify behaviour for "copy-from", "relative" and "proportional".

Also tested in-game using Saicchi's example in the linked issue:

{
  "id": "xl_armor_chitin",
  "type": "ARMOR",
  "name": { "str": "XL chitinous armor" },
  "copy-from": "armor_chitin",
  "proportional": { "weight": 1.125, "volume": 1.13, "price": 1.25 },
  "extend": { "flags": [ "OVERSIZE" ] },
  "armor": [ { "encumbrance": 10, "coverage": 90, "covers": [ "torso", "leg_l", "leg_r" ] } ]
}

Before

After

Additional context

See #43144 for more context about "proportional" and "relative".

[catdach]

A possibly related issue is that "encumbrance" also cannot be altered via "proportional" or "relative"; as the "armor_acidchitin"("biosilicified chitin armor") tries to do. Though I suspect it's because encumbrance is part of the "armor" field now? So it might be an entirely separate issue.

[dseguin]

A possibly related issue is that "encumbrance" also cannot be altered via "proportional" or "relative"; as the "armor_acidchitin"("biosilicified chitin armor") tries to do. Though I suspect it's because encumbrance is part of the "armor" field now? So it might be an entirely separate issue.

That's a separate issue (well, multiple issues really):

1. "proportional" and "relative" don't handle stuff in nested objects (i.e.: "armor": []). That would have to be handled as an elaborate special case.
2. You can't copy-from individual "armor" objects since they don't exist as individual types, which is a requirement for "proportional" and "relative".