Using a `git_oid *` in filter options was a mistake; it is a deviation
from our typical pattern, and callers in some languages that GC may need
very special treatment in order to pass both an options structure and a
pointer outside of it.

Provide a mechanism to filter using attribute data from a specific
commit (making use of `GIT_ATTR_CHECK_INCLUDE_COMMIT`).

blob: fix name of `GIT_BLOB_FILTER_ATTRIBUTES_FROM_HEAD`

Adds info about initializing options with git_blob_filter_options_init

`GIT_BLOB_FILTER_ATTTRIBUTES_FROM_HEAD` is misspelled, it should be
`GIT_BLOB_FILTER_ATTRIBUTES_FROM_HEAD`, and it would be if it were not
for the MacBook Pro keyboard and my inattentiveness.

The `git_blob_filter_options_init` function should be included, to allow
callers in FFI environments to let us initialize an options structure
for them.

Removes doc comment on `git_blob_filter_options.version`, moves
information to `git_blob_filter_options` doc comment to remain
consistent with other options structures' documentation.

`git_blob_filter_options_init` still needed; should be added in another
commit/PR (it's out of the scope of this PR, #5759), update this
documentation again.

Resolves #5756