Fix IndexedAttestation decoding

[jtraglia]

📝 Summary

When reviewing builder types, I noticed that the json encoding for AttestingIndices is missing the string option. The beacon API states (see reference for example) that this should be a list of uint64 strings.

go-boost-utils/types/builder.go

Line 62 in 417a646

AttestingIndices []uint64 `json:"attesting_indices" ssz-max:"2048"` // MAX_VALIDATORS_PER_COMMITTEE

I was expecting to see something like json:"attesting_indices,string" but I know that won't work.

Other uint64 fields are told to be strings, for example:

go-boost-utils/types/builder.go

Line 53 in 417a646

Slot uint64 `json:"slot,string"`

I made a test to verify my theory. I've included it here. Keep in mind it will currently fail with this message:

make test
go test ./...
ok  	github.com/flashbots/go-boost-utils/bls	(cached)
--- FAIL: TestIndexedAttestation (0.00s)
    builder_test.go:231:
        	Error Trace:	builder_test.go:231
        	Error:      	Received unexpected error:
        	            	json: cannot unmarshal string into Go struct field IndexedAttestation.attesting_indices of type uint64
        	Test:       	TestIndexedAttestation
FAIL
FAIL	github.com/flashbots/go-boost-utils/types	0.114s
FAIL
make: *** [test] Error 1

It will pass if you change attesting_indices from this to that:

"attesting_indices": [
        "1", "2", "3"
      ],

"attesting_indices": [
        1, 2, 3
      ],

Opening as a draft because I'm not entirely sure what the proper fix it. Would like to discuss.

⛱ Motivation and Context

There's a minor problem with IndexedAttestation encoding/decoding that should be fixed.

📚 References

• https://ethereum.github.io/beacon-APIs/#/Beacon/getPoolAttesterSlashings
• https://stackoverflow.com/questions/49415573/golang-json-how-do-i-unmarshal-array-of-strings-into-int64

---

✅ I have run these commands

• make lint
• make test
• go mod tidy

[jtraglia]

I think we need to add a U64Str type to common.go with MarshalText, UnmarshalJSON, and UnmarshalText methods. I've spent a little time trying to figure this out, but I can't seem to get it working. Since this isn't that straight-foward, I'm going close this PR, make an issue, and let someone else figure out how to fix it.

[metachris]

Reopening this PR because the test is useful and we can reuse that in the proper solution.

[metachris]

See also #18