refactor list splits pagination, env var to skip initial seeding

[mattmkim]

Description

• we were paginating list metrics/sketch splits with 3 different implementations, puts it in one place
• adds env var to skip intial seeding/fetching of immature splits on startup. TODO is to rewrite the query to only fetch splits we should care about (given some node_id, within the maturity period)

How was this PR tested?

Describe how you tested this PR.

[chatgpt-codex-connector]

💡 Codex Review

https://github.com/quickwit-oss/quickwit/blob/8a348274c5409a6aaf0804a2d630208a31ac4685/quickwit-indexing/src/actors/metrics_pipeline/mod.rs#L15-L20
Restore the metrics pipeline module declarations

This file now declares metastore submodules such as file_backed, index_metadata, and control_plane_metastore, but the quickwit-indexing/src/actors/metrics_pipeline directory only contains the Parquet pipeline files (parquet_doc_processor.rs, pipeline.rs, etc.; I checked the directory and searched for pub mod file_backed). As soon as the metrics pipeline module is compiled, Rust will look for missing files like metrics_pipeline/file_backed.rs while the real pipeline submodules and re-exports were removed, so metrics indexing cannot build or expose ParquetMergePipeline, MetricsPipeline, and related actors.

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[g-talbot]

Didn't look super deeply but this seems like a worthwhile refactor.