Change IBeamIntensity API to take run and event number explicitly

• The IBeamIntensity API now takes the run and event number explicitly.
• StepArrayBM and ArrayBM now use ContextSpecificData for the t0 offset. SlotSpecificObj is not used because this also needs to work when there is no valid context
• ArrayBM now has a Seed property. If this is set to a non-zero value, it uses FastReseededPRNG seeded with this value and the run and event number instead of the AthRNGSvc
• FastReseededPRNG has been added to CxxUtils. For now xxhash is vendored, until it can be added to externals or the LCG release

Control/CxxUtils/CxxUtils/FastReseededPRNG.h

+/*
+  Copyright (C) 2023 CERN for the benefit of the ATLAS collaboration
+
+----------------------------------------------------------------------
+Based on Xoroshiro code with copyright notice:
+Written in 2018 by David Blackman and Sebastiano Vigna (vigna@acm.org)
+
+To the extent possible under law, the author has dedicated all copyright
+and related and neighboring rights to this software to the public domain
+worldwide. This software is distributed without any warranty.
+
+See <http://creativecommons.org/publicdomain/zero/1.0/>.
+----------------------------------------------------------------------
+
+This provides a uniform random bit generator using xoroshiro128**
+seeded using 128 bit XXH3 to hash the provided seeds.
+
+Compared to other URBGs this is very cheap to construct, which is useful
+when the PRNG is reseeded every event and only used to generate a few random
+numbers.
+*/
+#ifndef PILEUPMT_FASTRESEEDEDPRNG_H
+#define PILEUPMT_FASTRESEEDEDPRNG_H
+#include <array>
+#include <cstdint>
+#include <limits>
+
+class FastReseededPRNG {
+ public:
+  using result_type = std::uint64_t;
+  template <typename... Int>
+  FastReseededPRNG(Int... seed) {
+    const std::array<std::uint64_t, sizeof...(seed)> seed_array{
+        static_cast<std::uint64_t>(seed)...};
+    set_seed(seed_array.data(), seed_array.size());
+  }
+  FastReseededPRNG() = delete;  // No default constructor. Always need a seed.
+
+  static constexpr result_type min() { return 0; }
+  static constexpr result_type max() {
+    return std::numeric_limits<std::uint64_t>::max();
+  }
+
+  result_type operator()();
+
+ private:
+  std::array<std::uint64_t, 2> m_seed_arr;
+  void set_seed(const std::uint64_t* start, std::size_t len);
+  std::uint64_t next();
+};
+
+#endif  // PILEUPMT_FASTRESEEDEDPRNG_H