Today the way we're pursuing people building signing plugins is to build CLIs that accept a transaction envelope as an input and emit a transaction envelope as output. For example:

• https://github.com/stellar/soroban-examples/blob/8cb0f69cce36e083687a415ef8c3859eec111d4b/multisig_1_of_n_account/stellar-cli-sign-auth-ed25519/src/main.rs

We should explore other ways for folks to build signing plugins that operate on the basic primitives of the SorobanAuthorizationEntry, it's contained invocation and credential.