Add get_live_bytes() to report the current heap footprint of theta, tuple and kll sketches

[stojkomilos]

What

Adds a public get_live_bytes() accessor that reports, in O(1), the exact heap a sketch's
mutable internal state is holding right now. The sketches already expose
get_serialized_size_bytes(), but that is an upper bound on the serialized form; there is
currently no way to ask how much memory a live, mutable sketch is actually retaining. This is
useful for memory accounting when many mutable sketches are held in memory at once (e.g. one
sketch per group/window in a streaming aggregation).

Where

• theta / tuple — the accessor lives on theta_update_sketch_base, the hash table shared by
  theta and tuple update sketches, unions and intersections. It returns
  (1 << lg_cur_size_) * sizeof(Entry), or 0 when the table is unallocated (lg_cur_size_ == 0
  leaves entries_ == nullptr). update_theta_sketch, theta_union, theta_intersection, the
  tuple update sketch, tuple_union and tuple_intersection forward to it. Tuple's union and
  intersection reuse the theta base as their State, so no separate implementation is needed
  there.
• kll — not a hash table, so kll_sketch::get_live_bytes() sums the items_ buffer
  (items_size_ elements) and the levels_ vector, mirroring the sketch's allocate() sites.

Unlike get_serialized_size_bytes(), this touches no data and does no iteration; it just reports
the current allocation size.

Tests

Adds Catch2 cases in theta_sketch_test, theta_union_test, theta_intersection_test,
tuple_sketch_test and kll_sketch_test that:

• check the footprint is a power-of-two count of entries where applicable,
• confirm an unallocated intersection reports 0,
• confirm the footprint only grows (never shrinks) as entries are inserted and ends up larger than
  the initial allocation.

All theta / tuple / kll suites pass locally.

Notes

Scope is theta, tuple and kll. hll and cpc could get an equivalent accessor in a follow-up if
maintainers want the whole family consistent.