[BE] Add /api/fluxshardsets endpoints

[foot]

That queries and returns fluxshardsets

What info to return here?

• See designs in [UI] flux-shard-controller UI changes for OSS #2888

Worth spending a timeboxed half a day trying to use the explorer api for this! See

• https://github.com/weaveworks/weave-gitops-enterprise/blob/main/docs/extending-explorer.md

If not working can use /api/gitopssets etc as a good example instead.

Sub tasks

• add flux-shard-controller/api to go.mod etc as a dependency so we query for them etc.