[Feature Request] Secret support for celld

So we don't have to put secret in wrangler.json

Expected command would be `celld secret put XXX` (like wrangler cli)
(And maybe a local overrides(.dev.vars) for local dev)

Start by locating celld’s CLI command dispatch and configuration handling, then inspect how deployed values are currently read from wrangler.json. Define the behavior for `celld secret put XXX` and the optional `.dev.vars` local override; done means secrets no longer need to be placed in wrangler.json and local development behavior is covered.

Written by the indexing model from the issue text.