[NIT-2774] Enable external-signer for batch-poster with DA

[gligneul]

When running a batch-poster with anytrust, Nitro needs to load the batch-poster parent-chain wallet config to sign the DA messages even if an external signer is also being used.

Close NIT-2774

[gligneul]

Since this change is on a configuration on the nitro binary, it doesn't make sense to add a system test. I will test it manually with the nitro test node.

[eljobe]

LGTM

[gligneul]

I tested this on the nitro test node using the code in the Anytrust PR and a mock external signer that I hacked based on the external signer unit test. In this configuration, I set both the node.batch-poster.data-poster.external-signer and node.batch-poster.parent-chain-wallet config options. (The Anytrust options were also set based on the Anytrust PR). I spun up the node, sent a transaction, and confirmed that the node posted a batch using the external signer.

I also created a follow-up ticket on Linear to add an external-signer option to the nitro test node.