Refactor library RPC implementation of `admire::register_adhoc_storage`

The actual implementation of the RPC should be moved to admire::detail in impl.[ch]pp similarly to admire::register|update|remove_job().

This should include:

• Input arg job type should be changed to admire::job.
• Input arg ctx type should be changed to admire::adhoc_storage::context.
• Output arg adhoc_storage should disappear. Instead, an instance of admire::storage should be returned by the function.
• Since errors can occur and there's an output arg (adhoc_storage), the return type of the new function should be a tl::expected<admire::storage, admire::error_code>.
• Updating the C++ examples to fit the new API.

This task also probably requires:

• Redefining the RPC types for the RPC defined in rpc_types.[ch]pp
• Using conversion types to manage RPC types without memory leaks.