Generic remote-port adapter: implement any module protocol over HTTP.

Cross-module calls go through a protocol, so slicing a module into its own
process needs one thing the codebase did not have — an implementation of
that protocol that makes a network call. This builds one for an arbitrary
protocol, so the caller keeps calling `(ports/create-checkout-session svc …)`
and does not learn that the answer now comes over a socket (BOU-90).

Modelled on libs/external's outbound adapters: clj-http, `:throw-exceptions
false`, errors as data rather than exceptions.

FC/IS: shell. The wire contract is pure and lives in
`wagoe.platform.core.rpc`.

Expose a module's protocol over HTTP, so another process can call it.

The counterpart to `…rpc.client`: that turns protocol calls into requests,
this turns requests back into protocol calls against the local
implementation. Together they are what lets a module run as its own service
without its callers changing (BOU-90).

FC/IS: shell. The envelope contract is pure and lives in
`wagoe.platform.core.rpc`.