Summary:
Add an opaque external_table_config option to the external table adapter. RocksDB persists the string in OPTIONS files and passes it to ExternalTableFactory::Configure during bootstrap, allowing implementations such as Nimble to parse implementation-specific settings without adding RocksDB-specific fields. The option is immutable after DB open, and attempted runtime updates fail safely for non-clonable table factories. A reopen test verifies that callers can supply a different valid configuration between DB opens. Initialize the adapter option metadata lazily as const to avoid namespace-scope dynamic initialization and mutable-global lint issues.
PR https://github.com/facebook/rocksdb/pull/15119
Reviewed By: anand1976
Differential Revision: D116362444
fbshipit-source-id: 225a4aedc455f320b5c90ac2f0862917da477ccb
db1adf47bb