Consider making `aws_lambda_events::encodings::Body::Text` a `Cow<'static, str>`

[FalkWoldmann]

This would reduce the amount of allocations needed when the body content is known at compile time

[FalkWoldmann]

After trying it out a bit, unfortunately this doesn't look like it's that easy to do since impl<'a> From<&'a str> for Body does not live long enough. We could implement something like from_static_string but that would be quite uneconomic