Allow writes during SST file ingestion

[hhwyt]

Address #14137.

Summary: This change adds an allow_write option to IngestExternalFileOptions to allow writes to the DB during file ingestion. Previously, ingestion always blocked writes by entering write threads. With this option enabled, write operations can proceed concurrently with ingestion.

The main changes are in DBImpl::IngestExternalFile to conditionally skip blocking write threads based on the allow_write flag:

• Skip EnterUnbatched on write threads when allow_write=true
• Pass !allow_write as entered_write_thread to flush operations
• Skip WaitForPendingWrites when allow_write=true
• Skip ExitUnbatched when allow_write=true

Additionally:

• Add validation to ensure consistent allow_write across all ingestion arguments in a batch
• Update IngestExternalFileOptions with allow_write field (default false)

Users must ensure no writes overlap with the ingested data when using this option. This is useful for maintaining write availability during bulk data ingestion operations in scenarios where the ingested data does not conflict with ongoing writes.

Test Plan: Updated all existing test cases in external_sst_file_test.cc to support the new parameter with default value (false). All 202 tests pass successfully.

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