feat: code generate instruction account structs

[sol-mocha]

Description

This PR updates the code gen in main.go to create structs for instruction accounts, as well as a helper to populate said structs given the AccountMetaSlice in the instruction builder. This will allow devs to safely access instruction accounts in a readable way without having to write their own converters and types.

Open Questions

• How should we handle optional accounts in both the struct and in the function to populate the struct?

Example

idl/examples/composite.json now produces the following extra code

[
        {
          "name": "foo",
          "accounts": [
            {
              "name": "dummyA",
              "isMut": true,
              "isSigner": false
            }
          ]
        },
        {
          "name": "bar",
          "accounts": [
            {
              "name": "dummyB",
              "isMut": true,
              "isSigner": false
            }
          ]
        }
]

type CompositeUpdateAccounts struct {
	Foo struct {
		DummyA ag_solanago.PublicKey
	}
	Bar struct {
		DummyB ag_solanago.PublicKey
	}
}

func (inst *CompositeUpdate) GetCompositeUpdateAccounts() *CompositeUpdateAccounts {
	res := &CompositeUpdateAccounts{}
	res.Foo.DummyA = inst.AccountMetaSlice[0].PublicKey
	res.Bar.DummyB = inst.AccountMetaSlice[1].PublicKey
	return res
}

[sol-mocha]

@gagliardetto wondering if you can give some insight into the open question I have in the PR description. Do you anticipate any issues with the new code in relation to optional accounts?

[sol-mocha]

Here is a real life example using these changes. Its applied on the drip program idl.

[gagliardetto]

Hey @dcaf-mocha

Sorry for the delay in getting back to you.

Thanks for the PR! That's a great idea.

From what I remember:

• there can be optional accounts
• there can be groups of accounts
• there can be an arbitrary number of accounts (unnamed) at the end of the account list (I think they're the "remaining" accounts in anchor)