Use message history instead of event payload for conversation handler

Problem

Conversation handler receives messages in the event. The problem is that event size is constrained to 256 KB for asynchronous invocations. https://docs.aws.amazon.com/lambda/latest/dg/gettingstarted-limits.html .

When conversation is long enough and/or contains images this limit is hit and lambda can't be called.

Previous logic as defined in upstream resolver can be seen here. Note that it's using outdated gql schema.

Retrieve message history by:
1. Querying Appsync by conversationId and currentMessageId to recreate history.
2. Reconciling the history to pair user-asistant messages and drop unanswered questions.
3. Inject aiContext.
Keep event.messages for now to be backwards compatible until upstream components absorb this change. This will be removed later.
Tests.
Extract GraphqlRequestExecutor in the ai-constructs since we've hit the rule of 3 and refactor other components that execute GQL queries to use it.

Added tests, including e2e tests.

Manual tests via snapshot release with upstream components.

[ ] If this PR includes a functional change to the runtime behavior of the code, I have added or updated automated test coverage for this change.
[ ] If this PR requires a change to the Project Architecture README, I have included that update in this PR.
[ ] If this PR requires a docs update, I have linked to that docs PR above.
[ ] If this PR modifies E2E tests, makes changes to resource provisioning, or makes SDK calls, I have run the PR checks with the run-e2e label set.

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