Resolve "Feat: initial support for functions in the query builder API"

Closes #11 (closed) Closes #10 (closed)

Note: There will be duplicate fns as PromQL and MetricsQL both share some of them but they slightly differ in the number of args they support.

TODO

• Types and structure to represent fns
• Functions
  • PromQL aggregate operators as fns
    • sum
    • min
    • max
    • avg
    • group
    • stddev
    • stdvar
    • count
    • count_values
    • topk
    • bottomk
    • quantile
  • PromQL Rollup over time fns
  • Most internally used MetricsQL fns
    • label_map (closes #10 (closed))
    • sort_by_label
    • limit_offset
    • keep_metric_names (not a function)
    • union
• MetricsQL feature gate
• Integration tests
• Module level docs