Add static_map heterogeneous lookup example

[cnaples79]

Summary

• add a static_map example that demonstrates hashing and comparing compatible key types
• show how the map can answer contains and find queries using probe keys with extra fields
• register the example with the examples CMake target so it builds alongside existing samples

Rationale

• addresses [ENHANCEMENT]: Add an example for performing a heterogeneous lookup #714 by giving users a reference for heterogeneous lookups without overloading the existing mapping-table example

Changes

• new static_map/heterogeneous_lookup_example.cu with custom hash/equality for tuple probes
• build system entry so the sample is produced with the rest of the examples
• Tests not run (examples)

Fixes#714

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[PointKernel]

/ok to test 3d944b2

[PointKernel]

Looks good!

I’ve updated the README accordingly and did a bit of cleanup with some minor fixes. Thanks for the contribution, @cnaples79.