enhancement(lua transform): Allow iterating over event fields

[steven-aerts]

Make it possible to use pairs(event) to iterate over all fields of an event.

For example:

# Remove all fields where the value is "-"
for f,v in pairs(event) do
  if v == "-" then
    event[f] = nil
  end
end

This code will only compile after rlua takes in mlua-rs/rlua#152 as the __pairs meta-method is currently not exposed.

This commit fixes #528.

[binarylogic]

Thanks @steven-aerts! We'll get this reviewed.

[ghost]

Looks good to me.

This code will only compile after rlua takes in mlua-rs/rlua#152 as the __pairs meta-method is currently not exposed.

As it is still not merged, we can create a fork with changes from mlua-rs/rlua#152 and use rlua crate from it.

I also had troubles building Vector with master version rlua because it adds additional Into trait implementation for &str, which makes one of splunk_hec tests fail to compile. However, it can be fixed separately as part of switching to rlua crate from the fork.