feat: creation of a new interface for combined target + client installation

[erunion]

🧰 Changes

In @readme/oas-to-snippet right now the way we're installing httpsnippet-client-api is the following:

addTargetClient('node', HTTPSnippetSimpleApiClient);

This works normally but because we'd love to have folks create their own client plugins and give us NPM packages we don't want to have to supply a target for these plugins when it's easier if the plugin can define that themselves.

The work I've done here is to create a new addClientPlugin hook for installing a one of these target + client combinations and then also a new ClientPlugin interface that combines the two so we can enforce that plugins are properly defined:

interface ClientPlugin {
  client: Client;
  target: TargetId;
}

With all this work we'll be able to now execute the folllowing to install httpsnippet-client-api (once it's been converted to this new system of course):

addClientPlugin(HTTPSnippetSimpleApiClient);