feat: add Amazon Lex V2 event support (#518) - #615

Does Lex allow non-UUID session IDs? If not, we could replace this with a placeholder value that would represent something legal.

Lex V2 accepts any string for sessionId, I'll update this in the next commit to be similar to a UUID format https://docs.aws.amazon.com/lexv2/latest/dg/lambda-input-format.html
image

Would recommend a data type for InvocationSource:

// InvocationSource represents the source of the invocation within the Lex Bot lifecycle.
type LexInvocationSource string
const (
	LexInvocationSourceDialogCodeHook InvocationSource = "DialogCodeHook"
	LexInvocationSourceFulfillmentCodeHook InvocationSource = "FulfillmentCodeHook"
)

Would recommend a data type for InputMode:

// InputMode represents the mode of innput from the user.
type InputMode string
const (
	InputModeText InputMode = "Text"
	InputModeVoice InputMode = "Voice"
	InputModeDTMF InputMode = "DTMF"
)

intentionally kept these as plain string types to match the pattern used throughout the aws-lambda-go events library (see lex.go, cognito.go, apigw.go)

AWS can add new enum values without requiring library updates. Developers are pointed to the official AWS docs for valid values if they want to add type-safe constants in their own code.

https://docs.aws.amazon.com/lexv2/latest/dg/lambda-input-format.html
https://docs.aws.amazon.com/lexv2/latest/dg/lambda-response-format.html

This approach keeps the library minimal while remaining flexible as AWS evolves the service.

The LexV2Event struct is missing the optional InvocationLabel:

The documentation does not describes this field as optional, but testing has shown that the field is optional, so it can be represented as a string pointer.

Is InputTranscript actually optional? If so, it could be a pointer, so that the json struct tag omitempty, results in a nil value instead of a default empty string.

thanks will add InvocationLabel in next commit

InputTranscript: The documentation not explicitly mark inputTranscript as required. However, I'm keeping it as a regular string with omitempty because it represents user input and is present in all standard DialogCodeHook and FulfillmentCodeHook invocations. It's a core field fundamental to Lex's operation. Making it a pointer would incorrectly signal it's rarely used, when it's present in nearly every event. The omitempty tag handles edge cases where it might be absent. While the docs don't say "required" verbatim, the field's purpose and typical usage patterns indicate it's a standard field rather than optional.