Make buffer size for agent data configurable

[estolfo]

Right now the size of the buffered channel for agent data is 100. We should make it configurable with some factor. n * 5, for example.
Ref: https://github.com/elastic/apm-aws-lambda/blob/main/apm-lambda-extension/main.go#L50

• test what happens if buffer is full

[simitt]

Do we have a better understanding already about what happens when the buffer is full?