[Store] Structure OffsetAllocatorBackendConfig environment settings (#3408)

This PR uses OffsetAllocatorBackendConfig::FromEnvironment() as a focused pilot for consolidating environment configuration. These existing settings configure the offset allocator storage backend's eviction, capacity, persistence, and record-integrity behavior.

Add a typed parser for converting strings to supported value types, independent of environment lookup.
Add typed environment-variable definitions, group the offset allocator backend's existing variables together, and keep its legacy ratio parsing explicit in that group.
Route the group through Environ while keeping policy and persistence-mode mapping in the Store layer.
Preserve existing defaults, parsing and validation behavior, diagnostics, configuration precedence, and read timing.
Cover unset, valid, invalid, and explicitly empty values.
This PR does not change config-file or CLI handling, add dynamic configuration reloads, or change user-facing configuration semantics.

Signed-off-by: Schatten <czhengt@qq.com>