Define PlutusV4 script context

This ticket is created to track progress on PlutusV4 context that will include at least two CIPs:
* [CIP-112 - Observe script type](https://github.com/cardano-foundation/CIPs/pull/749)
* [CIP-118 - Nested Transactions](https://github.com/cardano-foundation/CIPs/pull/862)

Considering that CIP-112 will be required for CIP-118 it makes sense to start with it first.

* [ ] - Define `PlutusV4` version in `plutus-ledger-api`: https://github.com/IntersectMBO/plutus/blob/fa5c9e64ec395661eff8f6a5ce3d784e1100833c/plutus-ledger-api/src/PlutusLedgerApi/Common/Versions.hs#L89-L92
* [ ] - Create `V4` in `plutus-ledger-api:PlutusLedgerApi` that reuses as much stuff from V1-V3 as possible
* [ ] - Define `TxInfo` for PlutusV4
* [ ] - Add `txInfoGuards :: [Credential]` to `TxInfo` for PlutusV4 field and remove `txInfoSignatories`, since former supersedes the latter (Note that we will be using the term Guards in the implementation instead of Observers as CIP-112. CC @colll78 if you are still up to renaming the CIP to avoid confusion)
* [ ] - Add `Guarding` script purpose
* [ ] - Remove `PointerAddress` case for PlutusV4. See https://github.com/IntersectMBO/cardano-ledger/issues/4832 for reference
* [ ] - CIP-118 related parts added to TxInfo. See this https://github.com/IntersectMBO/plutus/issues/7342#issuecomment-4799132923 and this https://github.com/IntersectMBO/plutus/issues/7342#issuecomment-4852267096

I'd recommend creating a child ticket for each of the bullet points, so the progress can be tracked at more granular level, but I'll leave this up to the Plutus team.