settings: Expose GIT_OPT_SET_OWNER_VALIDATION option

[alexjfisher]

By default, libgit2 will throw an error if you try to open a repository not owned by the current user. An option to disable this validation is available, but wasn't being exposed in rugged.

This commit exposes fetching and setting this option via a new owner_validation setting.

(My use case is using rugged in a ruby language AWS lambda function connected to EFS. In this scenario, the EFS access point is configured with a POSIX user ID and group ID used for all file system operations using this access point, but the lambda's execution environment's uid is not known upfront and won't match).