Add FieldType.LONG and use it for the `size` field in BLOB_STORAGE

[mchades]

Context

In PR #47 (BLOB_STORAGE backend), the size field is defined as FieldType.INTEGER. However, file sizes can exceed the 32-bit integer range (2^31 - 1 ≈ 2.1 GB), so a LONG (64-bit integer) type would be more appropriate.

Proposed Changes

1. Add LONG to the FieldType enum in the protocol types.
2. Update the size field definition in src/backends/blob_storage/__init__.py to use FieldType.LONG.
3. Ensure all backend implementations handle the LONG type correctly (e.g., RDBMS mapping to BIGINT).

References

• PR [#46] feat(backends): add BLOB_STORAGE backend with local filesystem provider #47 review comment by @jerryshao: [#46] feat(backends): add BLOB_STORAGE backend with local filesystem provider #47 (comment)