Create new chrome-untrusted WebUI for displaying NFTs

[nvonpentz]

We added an API to support fetching of NFT metadata in brave/brave-core#12594 and have frontend code ready to display to display the images in brave/brave-core#13217 (since reverted). But before we merge all that in, we need to ensure all images sourced from the blockchain are rendered in sandboxed contexts. To that end we should add a new chrome untrusted WebUI for displaying NFTs.

From @petemill and @muliswilliam

• Create a new chrome-untrusted C++ webui and add it to BraveUntrustedWebUIControllerFactory
  • Follow one of the examples in that factory, e.g. UntrustedTrezorUI since it is also wallet
  • Give it a hostname, e.g. nft-image so it renders at chrome-untrusted://nft-image
• Make that webui serve a basic html page
• Have that html page (and a bit of TS if neccessary) display the image
• Put an iframe in the original wallet page that loads the chrome-untrusted://nft-image/xyz page in the iframe