Add `init` field to account IDL

[armaniferrante]

Filing a tracking issue, though I don't love the idea at the moment and am inclined to throw this idea away.

IDL change and client generation

An additional feature the #[account(init)] attribute enables is the ability for clients to know when they should create accounts in the same transaction as executing a program instruction.

Currently, when executing this type of instruction, clients must manually add the account creation instruction as follows.

// The Account to create.
const myAccount = new anchor.web3.Account();

// Atomically create the new account and initialize it with the program.
await program.rpc.myInstruction(...args, {
  accounts: {
    myAccount: myAccount.publicKey,
  },
  signers: [myAccount],
  instructions: [
    await program.account.MyAccount.createInstruction(myAccount),
  ],
});

With the #[anchor(init)] attribute, this example can be transformed into

await program.rpc.myInstruction(...args);

Since the client generator can read a new isInit field in the IDL

// idl.json
{
...,
instructions: {
  ...,
  {
    "name:" "myInstruction",
    "accounts": [...],
    "isInit": true,
    "isMut": true,
    "isSigner": true,
     ...
   }
}

[anoushk1234]

@armaniferrante should i take this up? I was building a transpiler for anchor to seahorse and this was a problem as it doesnt know which accounts are being initialized