add warning about file-backed storage

[JerryKwan]

add waning when using file-backend metastore add waning when using file-backend index

Description

Fix the issue https://github.com/quickwit-oss/quickwit/issues/1990 Describe the proposed changes made in this PR.

How was this PR tested?

Describe how you tested this PR. When using file-backed meta store it will output messages like the following 2023-09-15T03:36:37.522Z WARN quickwit_config::node_config::serialize: metastore_uri = file:///root/workspace/quickwit/quickwit/quickwit/data/indexes#polling_interval=30s. Using file-backed metastore comes with certain limitations, please refer to the specific documentation for details.

Closes #1990