[NRBF] change StartsWithPayloadHeader to accept a Span rather than array

[adamsitnik]

During the API Review I was asked why this method accepts an array rather than a readonly span of bytes. I've answered that because it's impossible to implement it for NS2.0 due to lack of primitives for endianness conversion. Back then I did not know that BinaryPrimitives.ReverseEndianness is supported on older frameworks.

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.