Add BlockBody::transaction_count helper function

[mattsse]

Describe the feature

ref #13626

we often use:

it be nicer to have this

• part of the blackbody trait, similar to feat: add helper iter for tx hashes #13638
• and exposed on SealedBlock as function that delegates to this trait function

TODO

• introduce these two functions (trait + native on SealedBlock)
• simplify .body().transactions().len() with .transaction_count

cc @Nanle100

Additional context

No response

[frankudoags]

Can I take this?

[mattsse]

assigned, ty

[mymiracle0118]

I’d love to work on this task.

[aidenwong812]

Could I try solving this?