Update CORS headers based on compatibility sRFC

[tsmbl]

Context & Problem

Based on sRFC https://forum.solana.com/t/srfc-31-compatibility-of-blinks-and-actions/1892

Response headers are proposed to be set by Action developers:

• X-Action-Version to show what spec version the action API server is using
• X-Blockchain-Ids to list blockchains the action supports.

In the future request headers are proposed to be set by Blink clients:

• X-Accept-Action-Version to show the max spec version the Blink client supports.
• X-Accept-Blockchain-Ids to list blockchains the client supports.

Wallets cannot get custom response headers like X-Action-Version and X-Blockchain-Ids by default: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Access-Control-Expose-Headers

Solution

1. Add Access-Control-Expose-Headers: X-Blockchain-Ids, X-Action-Version to make compatibility metadata available to scripts running in the browser.
2. Update Access-Control-Allow-Headers to include X-Accept-Action-Version and X-Accept-Blockchain-Ids as a preparation step to support client request headers in the future.

[nickfrosty]

Great catch @tsmbl, thanks!