[don’t merge] [BLB] Heirloom Epic, support AlternateManaPaymentAbility for abilities

[ssk97]

This was a rather difficult card as it's the first AlternateManaPaymentAbility that applies to an activated ability rather than a spell, which required engine changes to support. In particular, I moved currentActivatingManaAbilitiesStep and related functions into StackObject rather than Spell.

[JayDi85]

[[Heirloom Epic]]

[github-actions]

Heirloom Epic - (Gatherer) (Scryfall) (EDHREC)

{1}
Artifact
{4}, {T}: Draw a card. For each mana in this ability's activation cost, you may tap an untapped creature you control rather than pay that mana. Activate only as a sorcery.

[xenohedron]

@JayDi85 can you describe your concern with this one? It's been waiting for six weeks

[ssk97]

One potential issue is that since it is a static ability modifying the activated ability, cards that copy activated abilities don't get the alternative cost effect. Not sure how to reasonably deal with that issue (though it's not exactly a common interaction, I don't think that should be blocking)