Add configurable subpath for HTTP API

[stevekinney]

The change made in #410 causes a few potential problems for integrating with the UI.

First, some users host UI Server at a subpath and set both the client-side and API routes.

route.SetAPIRoutes(e, cfgProvider, serverOpts.APIMiddleware)
route.SetUIRoutes(e, cfg.PublicPath, assets)

Secondly, not having a sub-path for the API routes means that they will likely conflict with client-side UI routes. The current implementation of the UI passes through all routes with /api/v1 to UI Server. If both the pages and the API routes have the same path, it will be trickier to correctly implement the passthrough functionality.

Suggested resolutions:

• The path for the API routes should be configurable.
• They should default to something (e.g. /api) and not be set at the root by default since—selfishly—we'd need this set for CLI, Docker, and Temporal Cloud implementations anyway.

[cretz]

The path for the API routes should be configurable.

I have opened temporalio/temporal#6212 (this is not done in this repo, it's part of the server that sets up the API)

The change made in #410 causes a few potential problems for integrating with the UI. [...] They should default to something (e.g. /api) and not be set at the root by default since

We do have stable roots of /namespaces, /cluster, and /system-info (opened #428 to handle one we missed). We should never add more than those three. We do not serve anything from the literal /, we just removed some unnecessary API path parts, not the whole thing.