batch+config: extend configuration to allow pool to open channels from/to a cohort of lnd nodes

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Over time, we have plans to being to decouple the roles of the capital provider from the routing node that actually allocates the capital on the network. In addition to this, a use case we want to support is a wallet author remotely purchasing channels for their user base. We can enable both of these features by extending the configuration of `poold` to allow it to connect to a _set_ of `lnd` nodes.

These nodes would only be used to open channels, with `lmd` getting its funds from a single backing `lnd` node. In order to implement this feature, we'll need to further abstract channel funding and channel shim creation, and also come up with a sane configuration format to specify the set of `lnd` nodes. The [upcoming lnd feature to allow custom macarons based on URIs](https://github.com/lightningnetwork/lnd/pull/4463) can be used to make this feature safer, by ensuring the capital provider only has the ability to open channels and register funding shims.

Start by examining the poold configuration and the existing channel funding and channel shim creation paths. Define how a set of lnd nodes is specified while funds continue to come from one backing node, and use the custom macaroon URI feature to constrain permissions; done means poold can open channels through the configured cohort.

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