Support updateRuntimeConfig() to update global runtime config

[pi0]

Context: #2404 (comment)

Sometimes it is useful to update runtime config in server runtime using name. Currently, it is only possible by mapping environment variables with the same (and known names)

Example:

// server/plugins/runtime-config.ts

export default defineNitroPlugin(() => {
 const configOverrides = { foo: { bar: "baz" } }; // load from any source
 updateRuntimeConfig(configOverrides);
 // any code in any file after this line will be affected
})

Discussion:

We should choose between two orders: (low to high):

1. bundled vars > programmatic overrides > environment variable
2. bundled vars > environment variable > programmatic overrides

2 makes sense in the way that manual updateRuntimeConfig is more explicit than implicit environment variables (more making sense in testing scenarios) but it also breaks 12 factor and can cause headaches for sys admins not being able to quickly control instance behavior.

I am thinking of using order 1 but allowing a way to opt out from the environment variable support (testing environments in case there is a chance of conflict)