Optimise the implementation of GetNestedType

In input-output-hk/ouroboros-network#2226, we introduced a new `BlockComponent`, `GetNestedType`, which requires reading bytes the first `n` bytes from a block. When this is used in combination with other `BlockComponent` that require reading from the same file, we currently would open and close the same file twice, instead of keeping it open.
We can statically figure out whether a `BlockComponent` requires reading from a file and use that information to open the file once, in a scope higher than the one that interprets the `BlockComponent` query.

Moreover, if the first 0 bytes are requested (likely to be the case for pure Shelley in the future), we don't have to open the file at all.