Add ShardPrefixOverride to support custom shard naming

[raviiitb]

Added ShardPrefixOverride in Options to support a custom prefix for shard names. At GitLab, currently, we send the repository ID in the field RepositoryDescription.Name. Now we want to support repo: filtering. For that, we need to send the repository name for the field RepositoryDescription.Name. But we want to keep the prefix name as ID because it will be a huge effort to change the prefix now. Our suggestion is to add a new option field ShardPrefixOverride, which we can use to send the ID.

Also added the command-line option -shard_prefix_override for the same.

[raviiitb]

@keegancsmith, could you please review this PR?

CC @binarymason @dgruzd