bucket: Add prefix filtering capability

[stefanprodan]

This PR adds a new optional field .spec.prefix to the Bucket API. When specified the prefix is used to filter the objects server-side when .spec.provider is set to generic, aws and gcp.

Fix: #693
Fix: #248

Testing

if you want to try out this feature on your own test cluster:

1. Install the latest Flux controllers

flux install

2. Apply the Bucket CRD from this branch

kubectl apply --server-side --field-manager=flux -f https://raw.githubusercontent.com/fluxcd/source-controller/bucket-prefix/config/crd/bases/source.toolkit.fluxcd.io_buckets.yaml

3. Deploy the source-controller build from this branch

kubectl -n flux-system set image --field-manager=flux deployment/source-controller manager=ghcr.io/fluxcd/source-controller:bucket-prefix-8e215264

[tiffanywang3]

Thank you @stefanprodan; using the updated source controller image + Bucket CRD, we were able to leverage the prefix filtering which reduced Bucket reconciliation time from ~2.5 min to ~6 seconds (generic provider).

Looking forward to this update getting included in next Flux release!