tasks: allow user to not set all configs

[nyonson]

Been testing out integration and realized it's a pain to have to set all un-used config settings to empty lists.

Example from rust-psbt where I don't want to set exact_features and features_with_no_std:

features_with_std = ["rand", "serde", "base64", "miniscript"]
features_without_std = ["serde", "base64"]
exact_features = []
features_with_no_std = []

[tcharding]

ACK 41ff135

[tcharding]

@apoelstra slow me down if you don't want me merging wildly here.

[tcharding]

15 minutes pushed, review, merged - all on a Saturday morning, like a boss.