feat: allow overriding platform.apiRuntime

[geoffrich]

Closes #48

Instead of introducing a separate config option to set a custom node version, this PR allows setting platform.apiRuntime via customStaticWebAppConfig

e.g.

// svelte.config.js
import adapter from 'svelte-adapter-azure-swa';

/** @type {import('@sveltejs/kit').Config} */
const config = {
	kit: {
		adapter: adapter({
			customStaticWebAppConfig: {
				platform: {
					apiRuntime: 'node:18'
				}
			}
		})
	}
};

export default config;

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