Add a `GetABI()` method to abigen v2 or expose `abi` as `ABI`.

[mpolitzer]

Rationale

The current implementation requires that:

1. either the generated binds share a package with their users; or that
2. users call ParseABI() themselves to access the abi.

Having abi exposed as ABI or a GetABI() method would simplify client code.

Here is a section of the documentation on ethereum website showing direct abi access during Storage construction in a way that other packages are not allowed to do (abi accessible).

	...
	// create a BoundContract instance to interact with the pending contract
	storageABI, _ := StorageMetaData.ParseABI()
	contract := Storage{*storageABI}
	instance := contract.Instance(conn, address)
	...

ref. https://geth.ethereum.org/docs/developers/dapp-developer/native-bindings-v2

Why should this feature exist?

Better ergonomics. Make the API simpler to use.

What are the use-cases?

Simplify ABI access. For instance when filling the topics for a filter query with multiple contracts.
Here is a hypothetical call whith it implemented.

	contractA := ContractA.NewContractA()
	contractB := ContractB.NewContractB()

	fq := ethereum.FilterQuery{
		Topics: [][]common.Hash{
			contractA.GetABI().Events[ContractA.ContractAFooEventName].ID,
			contractB.GetABI().Events[ContractB.ContractBBarEventName].ID,
		},
	}

Implementation

Do you have ideas regarding the implementation of this feature? Yes, super simple.
Are you willing to implement this feature? Sure, here is a public domain patch.

diff --git a/accounts/abi/abigen/source2.go.tpl b/accounts/abi/abigen/source2.go.tpl
index 3d98cbb70..737d78a7f 100644
--- a/accounts/abi/abigen/source2.go.tpl
+++ b/accounts/abi/abigen/source2.go.tpl
@@ -59,6 +59,10 @@ var (
 		abi abi.ABI
 	}
 
+	func (c *{{.Type}}) GetABI() abi.ABI {
+		return c.abi
+	}
+
 	// New{{.Type}} creates a new instance of {{.Type}}.
 	func New{{.Type}}() *{{.Type}} {
 		parsed, err := {{.Type}}MetaData.ParseABI()

[ChickenBenny]

Hi @mpolitzer, I'd like to work on this issue. I plan to implement the GetABI() method in the abigen v2 template as described. Could you let me know if this is still available to pick up?

[mpolitzer]

It is!

Can you check with core maintainers if they are ok with the feature?

[ChickenBenny]

Awesome, thanks! Since I'm a first-time contributor to this repo, I don't really know the core maintainers yet to ask them directly. Should we just wait for one of them to chime in on this thread, or is there someone specific we should tag to get their thoughts?

[mpolitzer]

Either contacting them via discord or open a PR directly since this is a small change.
Readme has the details, on the contribution section: https://github.com/ethereum/go-ethereum?tab=readme-ov-file#contribution