Missing walletPubKey function

[jeffhappily]

Area

[] Plutus Foundation Related to the GHC plugin, Haskell-to-Plutus compiler, on-chain code
[x] Plutus Application Framework Related to the Plutus application backend (PAB), emulator, Plutus libraries
[] Marlowe Related to Marlowe
[] Other Any other topic (Playgrounds, etc.)

Describe the feature you'd like

There used to be a function called walletPubKey in Wallet.Emulator.Wallet but it's being removed in this commit due to the change of WalletId type.

But now the definition of walletPubKeyHash is essentially converting the Wallet into a MockWallet and calculate the PubKeyHash, it should be possible to define a walletPubKey function.

https://github.com/input-output-hk/plutus-apps/blob/eabb8c01bea0f6f476c01dde89b408c397de3ed5/plutus-contract/src/Wallet/Emulator/Wallet.hs#L131-L137

[jeffhappily]

I can open a PR if you'd like