Add blake3 benchmark for merkledb

[StephenButtolph]

Why this should be merged

BLAKE3 is a popular alternative to SHA256 which may be able to be configured to out-perform SHA256. Related twitter convo.

goos: darwin
goarch: arm64
pkg: github.com/ava-labs/avalanchego/x/merkledb
Benchmark_SHA256_HashNode/empty_node-12         	14813816	        85.51 ns/op	       0 B/op	       0 allocs/op
Benchmark_SHA256_HashNode/has_value-12          	13826523	        85.75 ns/op	       0 B/op	       0 allocs/op
Benchmark_SHA256_HashNode/has_key-12            	16249858	        74.29 ns/op	       0 B/op	       0 allocs/op
Benchmark_SHA256_HashNode/1_child-12            	 8004384	       142.2 ns/op	       0 B/op	       0 allocs/op
Benchmark_SHA256_HashNode/2_children-12         	 5870755	       203.5 ns/op	       0 B/op	       0 allocs/op
Benchmark_SHA256_HashNode/16_children-12        	 1000000	      1052 ns/op	       0 B/op	       0 allocs/op
Benchmark_BLAKE3_HashNode/empty_node-12         	 3500679	       346.3 ns/op	       0 B/op	       0 allocs/op
Benchmark_BLAKE3_HashNode/has_value-12          	 3406614	       360.3 ns/op	       0 B/op	       0 allocs/op
Benchmark_BLAKE3_HashNode/has_key-12            	 3430132	       348.2 ns/op	       0 B/op	       0 allocs/op
Benchmark_BLAKE3_HashNode/1_child-12            	 2954095	       426.4 ns/op	       0 B/op	       0 allocs/op
Benchmark_BLAKE3_HashNode/2_children-12         	 2223614	       547.5 ns/op	       0 B/op	       0 allocs/op
Benchmark_BLAKE3_HashNode/16_children-12        	  660697	      1837 ns/op	       0 B/op	       0 allocs/op
PASS

How this works

Because golang generics aren't great... This copies the sha256 hasher implementation and uses a blake3 hasher. I'm not aware of a way to share this code without introducing memory allocations (as passing a buffer into an interface will introduce memory allocations and golang does not specialize generic functions for interfaces).

How this was tested

• Added unit tests + benchmarks.