[NBS] Allow custom BlockDigestGenerator for volumes

[sharpeye]

Introduce IBlockDigestGeneratorFactory to allow volumes to use a BlockDigestGenerator created from their effective storage configuration.

Previously, the block digest generator was created once during service bootstrap and shared by all volumes. This prevented per-volume storage configuration overrides from affecting block digest generation.

This PR:

• adds a default IBlockDigestGeneratorFactory implementation;
• moves the existing generator selection logic into the factory;
• propagates the factory through the storage service and volume session actors;
• creates and updates the volume-specific digest generator using the volume's effective storage configuration;
• preserves the existing behavior for disabled digests, the test generator, and the ext4 digest generator;
• updates the affected tests and test environments.

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🟢 linux-x86_64-relwithdebinfo target: cloud/blockstore/ (test time: 11938s): all tests PASSED for commit b137ab8.

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7305	7304	0	0	0	1	0

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🟢 linux-x86_64-relwithdebinfo target: cloud/blockstore/ (test time: 11802s): all tests PASSED for commit 8c5b418.

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Planned checks for linux-x86_64-relwithdebinfo.

• ✅ blockstore

🟢 linux-x86_64-relwithdebinfo target: cloud/blockstore/ (test time: 11812s): all tests PASSED for commit 0504c48.

TESTS	PASSED	ERRORS	FAILED	FAILED BUILD	SKIPPED	MUTED?
7305	7304	0	0	0	1	0