feat(k8s): external S3 for LakeFS blockstore & Lakekeeper warehouse

[aicam]

Feature Summary

PR #5932 made the application tier's object storage configurable via a storage.s3 block, but left the two data-tier components still hardwired to the in-cluster MinIO:

• LakeFS blockstore (dataset versioning) points at {{ .Release.Name }}-minio.
• Lakekeeper Iceberg REST catalog warehouse is initialized against MinIO, and its init job unconditionally creates the bucket and emits an s3-compat storage profile (custom endpoint + path-style access).

There is also no single switch to drop the bundled MinIO entirely, and no worked example of an all-external-S3 deployment. As a result a fully external-S3 install (e.g. AWS S3 on EKS) is not yet possible with the chart. This is the LakeFS/Lakekeeper half of "make object storage pluggable" that #5932 explicitly deferred.

Proposed Solution or Design

Finish the external-S3 story so an operator can run Texera with no in-cluster MinIO at all, backing the app tier, LakeFS, and Lakekeeper by an external S3 store:

• minio.enabled switch (values.yaml, default true) gating the bundled MinIO subchart (Chart.yaml dependency condition), its persistence template, and the MinIO ExternalName alias exposed to the computing-unit pool namespace. Default install renders identically.
• LakeFS external blockstore: keep the in-cluster MinIO blockstore as the default; document overriding lakefsConfig to a region-only S3 blockstore with credentials injected via extraEnvVars for external S3.
• Lakekeeper init job: route STORAGE_S3_* through the shared texera.s3.* helpers (added in feat(k8s): make app-tier object storage configurable via storage.s3 #5932); add lakekeeperInit.createBucket (skip mc mb for pre-existing external buckets whose IAM principal lacks CreateBucket); add warehouse.flavor (s3-compat vs native aws, the latter omitting endpoint + path-style access) and an optional warehouse.keyPrefix to isolate the warehouse within a shared bucket.
• values-aws.yaml example overlay: a complete, copy-pasteable all-external-S3 configuration (MinIO off, storage.s3, LakeFS S3 blockstore, Lakekeeper flavor: aws + createBucket: false).

Non-breaking: with no overrides the chart renders the same set of resources as today.

Affected Area

Deployment / Infrastructure, Storage / Metadata