Allow option to pass file mode when opening a PR

Proposal to pass file mode when opening a PR:

## Use case

PR containing a bash script that should have the executable flag set

## Proposal

Allow the `files` values to be either a string, `null` or an `object`.

If the value is an object, the object `must` contain the key `content` that can either be a string or null.

The object might contain the `mode` key that will override the default `100644` mode for the specific file.

## Implementation

I've already drafted a possible implementation in case you are willing to accept this change.

https://github.com/LucaLanziani/octokit-create-pull-request/pull/1/files