Add method to validate Shopify webhooks using `ReadOnlyMemory<byte>`

[nozzlegear]

Merging method contributed by @Socolin, at their request.

• Remove unnecessary allocation
• Remove conversion from string to bytes

Benchmark:

Method	Mean	Error	StdDev	Median	Gen0	Gen1	Allocated
Old	2.308 μs	0.0694 μs	0.2045 μs	2.392 μs	0.0057	0.0038	624 B
New	1.115 μs	0.0016 μs	0.0013 μs	1.115 μs	-	-	64 B
OldLarge	2.383 μs	0.0469 μs	0.0482 μs	2.384 μs	0.0114	0.0076	1088 B
NewLarge	1.245 μs	0.0022 μs	0.0018 μs	1.245 μs	-	-	64 B