Implement the Redirector interface on httpjson's InputManager so
that when run_as_cel is true and cel.program is present, the Loader
redirects to the cel input with a translated config.

convertHttpjsonToCel maps the full set of shared fields between the
two inputs: interval, auth, request.url to resource.url, retry,
redirect, keep_alive, tracer, and inline transport settings (ssl,
timeout, proxy_url, proxy_headers, proxy_disable,
idle_connection_timeout). cel-namespaced fields (program, state,
regexp, xsd, max_executions, redact) are extracted to their
top-level equivalents. Fields specific to httpjson's declarative
config (method, body, transforms, response, cursor, chain) are
excluded since the CEL program handles them directly.

max_executions uses Int/SetInt to match its *int type in the cel
config rather than round-tripping through strings.

Parse the URL through url.Parse before constructing the cursor
store key so that the key matches source.Name() normalization.
Without this, URL round-tripping through net/url could cause a
mismatch between the key we look up and the key the input wrote.

Add input.Name() assertion to TestRedirect_EndToEnd to verify the
Loader actually redirected to the cel plugin.