Function logs can be reported without `faas.execution` field set

**APM AWS Lambda extension version** (`main`):

**Describe the bug**
It is possible that function logs are reported without the `faas.execution` field set since the current code assumes that both `platform.start` and `function` logs for a given invocation will be reported in the same batch ([relevant code](https://github.com/elastic/apm-aws-lambda/blob/ea8baf48e6b0665bc251520606ec4629a00208b2/logsapi/event.go#L67-L70)).

**To Reproduce**
Steps to reproduce the behavior:
1. Run a lambda function generating function logs with the latest version of extension (built from main).
2. Invoke the function a bunch of times.
3. Observe the missing `faas.execution` field on Kibana for some logs.

**Expected behavior**
All logs should have `faas.execution` set. This being said, it is not always possible to have `faas.execution` especially if the `platform.start` and `function` logs are delivered to two separate instances which _I think_ should be possible.

**Provide logs (if relevant)**:

^ Snapshot showing missing `faas.execution` in logs.