Fix a bug in read options where the value of zero-len option is always
set to `true` even if the flag passed is false. This meant if the option
was present it would have always enabled the zero-len as EOF.

Allow the user to specify read options on `ReadOrGenerateIndex`. Pass
the options onto downstream reader/generators.

Generate a CARv1 file with null padding and check it into `testdata`. In
addition check in another such file from ignite just to have two
different samples of this case.

Enhance index generation tests to expect error when zero-len option is
not set, and generate index when it is for files with null padding.

Make RW blockstore test use the fixed test files from `testdata` instead
of generating every time.