[FEAT] Bucketing improvements in fractional targeting

[cupofcat]

• [P0] Support weights not summing up to 100
• [P1] Support a much higher resolution of buckets (e.g. be able to have a probability of 0,0001 of selecting a bucket)
• [P1] Support a much higher number of possible buckets (>= 10K)
• [P2] Handle missing bucketing keys gracefully

Related ADRs:
#1800