[spec] v2.3 - Optional Transaction In Action Chaining and External Link Support

[thearyanag]

TLDR

Currently, Actions can be chained together in successive series, but each action in the chain requires the user to sign a transaction thus creating a bad UX.

1. A Post Request Type ( Optional Transaction ) has been added to avoid signing the transaction on each action.
2. A External-Link type has been added, to direct the user to an external website.

This would enable developers to

• providing better UX by avoiding individual pop-ups for each transaction, with the help of optional transactions.
• support cases such as adding a Learn More or Manage your Squads kind of thing, with the help of an external link.
• Later can be extended to support use cases like sign-message where each flow on the blink client end will be slightly different.

Rationale

Blinks and actions currently require individual transactions for each action in the chain. If the user wants to move to the next action in the chain, they can't without signing a transaction. Having optional transactions would enable a better UX and open more design space for the developers.

sRFCs :

1. https://forum.solana.com/t/srfc-32-optional-transactions-in-action-chaining/1971/18
2. https://forum.solana.com/t/blinks-cta-external-linking/2018/10

[nickfrosty]

@thearyanag also, the NextActionPostRequest should be updated to make the signature optional too since with this PR, not all actions will be able to provide a signature