Add completion for the fish shell

[rombert]

Describe the problem/challenge you have

The velelro CLI tools currently provides completions for bash and zsh. It would be very useful to also provide completions for fish.

Describe the solution you'd like
velero completion fish should generate a fish completion script.

Anything else you would like to add:
No

Environment:

• Velero version (use velero version): 1.5.2
• Kubernetes version (use kubectl version): Not relevant
• Kubernetes installer & version: Not relevant
• Cloud provider or hardware configuration: Not relevant
• OS (e.g. from /etc/os-release): openSUSE Tumbleweed x86_64

Vote on this issue!

This is an invitation to the Velero community to vote on issues, you can see the project's top voted issues listed here.
Use the "reaction smiley face" up to the right of this comment to vote.

• 👍 for "The project would be better with this feature added"
• 👎 for "This feature will not enhance the project in a meaningful way"

[rombert]

Thanks a lot for including this!