It's clear in the rules that cards must be purchased by heroes who have a matching resource symbol, but can attachments be attached to any hero or must they be attached to a hero/ally with a matching resource symbol?

E.g. Does Steward of Gondor have to be attached to a Leadership hero or can it attach to any hero that player controls?

Self Preservation says "Attach to Character". I assume this means it can attach to a hero or an ally. Does the hero/ally have to have a matching resource symbol or will any hero/ally do?

You can play attachments on any hero, unless the card specifies otherwise. The resource matching is only for paying for the cost for the card.

You can even play attachments on another player's hero.

Yeah if it says character than it can be attached to a hero or ally. Also there are attachments for enemies like Forest Snare.

The existence of cards like A Burning Brand ("Attach to a [lore] hero") and the fact that other attachments do not make this specification should provide you with the answer to that question.