consider adding `toURL` method to embed best practices of serializing cid to URL

There had been various discussions about the fact that in the wild we see IPFS urls to specific gateways. I think we can bake a best practice into CID implementation by adding a method like:

interface CID {
  // ... stuff we already have
  /**
   * Encodes CID into a URL object.
   *
   * @example
   * ```js
   * const cid = CID.parse('QmbrRJJNKmPDUAZ8CGwn1WNx2C7xP4J284VWoAUDaCiLaD')
   * cid.toURL().href // 'ipfs://bafybeigizayotjo4whdurcq6ge7nrgfyxox7ji7oviesmnvgrnxn3nakni/'
   * ```
   * 
   * Optionally you could provide a gateway URL to encode CID to a URL in that gateway.
   * @example
   * ```js
   * const cid = CID.parse('QmbrRJJNKmPDUAZ8CGwn1WNx2C7xP4J284VWoAUDaCiLaD')
   * cid.toURL({ gateway: new URL('https://dweb.link') }).href
   * // => 'https://dweb.link/ipfs/bafybeigizayotjo4whdurcq6ge7nrgfyxox7ji7oviesmnvgrnxn3nakni'
   * ```
   */
  toURL(options?: { gateway?: URL }):URL
}

Note that here we reinforce several of the best practices:

1. ipfs://${cidv1} is the default that we to see.
2. We want to encourage CID v1 because it addresses bunch of issues that we have with v0.
3. Gateways URLs are origin separated.

Start with the CID interface and the two URL examples in the issue, then inspect the repository's existing CID implementation and tests to identify the appropriate entry point. Determine the expected behavior for default ipfs URLs, CID version handling, and gateway URLs; the issue is complete when those semantics are agreed and covered by tests.