Tezt/Client: update a helper function

What

Update Client.import_public_key type from

 val import_public_key :
  ?force:bool ->
  ?endpoint:endpoint ->
  t ->
  Account.secret_key ->
  alias:string ->
  unit Lwt.t

to

val import_public_key : ?force:bool -> ?endpoint:endpoint -> t -> string -> alias:string -> unit Lwt.t

Why

Unlike private keys, public keys are never encrypted. As a result, the only actual use case for this helper is to call Client.import_public_key alias client @@ Unencrypted pk, unnecessarily forcing test writers to wrap their public keys in the Unencrypted constructor.

How

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR