[Feature]: Allow creation of AppGenesis without a file lookup

[chatton]

Summary

Provide an alternative to AppGenesisFromFile which does not depend on a file lookup to create an instance of AppGenesis.

Problem Definition

In the ibc-go e2e tests, we use interchain test which provides a hook to modify the genesis bytes. In order to provide modifications, we need to serialize the bytes, make changes, and return the new bytes.

The current implementation of AppGenesisFromFile does a direct os.ReadFile and so is not quite flexible enough to meet our use case.

Proposed Feature

I propose the method signature is changed (or a new fn is added is the original behaviour is still required!)

I think using an io.Reader is a nice ux for this.

// AppGenesisFromReader reads the AppGenesis from the reader.
func AppGenesisFromReader(reader io.Reader) (*AppGenesis, error) {
	jsonBlob, err := io.ReadAll(reader)
	if err != nil {
		return nil, err
	}

	var appGenesis AppGenesis
	if err := json.Unmarshal(jsonBlob, &appGenesis); err != nil {
		// fallback to CometBFT genesis
		var ctmGenesis cmttypes.GenesisDoc
		if err2 := cmtjson.Unmarshal(jsonBlob, &ctmGenesis); err2 != nil {
			return nil, fmt.Errorf("error unmarshalling AppGenesis: %w\n failed fallback to CometBFT GenDoc: %w", err, err2)
		}

		appGenesis = AppGenesis{
			AppName: version.AppName,
			// AppVersion is not filled as we do not know it from a CometBFT genesis
			GenesisTime:   ctmGenesis.GenesisTime,
			ChainID:       ctmGenesis.ChainID,
			InitialHeight: ctmGenesis.InitialHeight,
			AppHash:       ctmGenesis.AppHash,
			AppState:      ctmGenesis.AppState,
			Consensus: &ConsensusGenesis{
				Validators: ctmGenesis.Validators,
				Params:     ctmGenesis.ConsensusParams,
			},
		}
	}

	return &appGenesis, nil
}

// AppGenesisFromFile reads the AppGenesis from the provided file.
func AppGenesisFromFile(genFile string) (*AppGenesis, error) {
	file, err := os.Open(genFile)
	if err != nil {
		return nil, err
	}
	return AppGenesisFromReader(bufio.NewReader(file))
}

[tac0turtle]

want to open a pr and we can include it in 0.50 since its not api breaking?

[chatton]

can do! 👍