Package batch balancing by weight

Currently distributing packages in to matrix legs or build/test batches (azure-sdk-for-net) legs are balanced by package count. This leads to a balance issue in leg time because of size, complexity or test count differences between the packages.

If we considered package "weight" when balancing legs, we could support balancing of leg work by adjusting the weight of each package.

By default, setting the weight of each package to 1 would result in legs balanced by package count. Allowing the weight to be programmatically set by the language repo per package would allow for balance strategies like line count, test count, file count, etc.

No file or test is named. Start by locating the code that distributes packages into matrix legs or build/test batches, then trace how package counts are calculated. Done means legs can be balanced by configurable package weight, with weight 1 preserving count-based behavior and language repositories able to provide other weights.

Written by the indexing model from the issue text.