feat: Move DefaultMetadataCache into its own module (#19125)

## Which issue does this PR close?
Implements #19074

## Rationale for this change
As the default cache implementations expand, the module structure under
`datafusion/execution/src/cache/` has become crowded. This PR continues
the pattern established in #18855 by separating
`DefaultFilesMetadataCache` into its own module to improve code
organization and maintainability.
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## What changes are included in this PR?
- Created new `file_metadata_cache.rs` module containing:
    - `DefaultFilesMetadataCacheState` (internal state management)
    - `DefaultFilesMetadataCache` (public struct and implementation)
- All trait implementations (`FileMetadataCache` and `CacheAccessor`)
    - All 3 related tests and test helpers
- Updated `mod.rs` to declare and export the new module
- Updated `cache_manager.rs` to import from new location
- Cleaned up `cache_unit.rs` (reduced from 863 to 159 lines)
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## Are these changes tested?

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Yes, all existing tests have been moved to the new module and pass
successfully

## Are there any user-facing changes?
No user-facing changes. This is purely an internal code organization
refactoring.
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Author: AryanBagade

datafusion/execution/src/cache/cache_manager.rs

@@ -16,7 +16,7 @@
 // under the License.
 
 use crate::cache::CacheAccessor;
-use crate::cache::cache_unit::DefaultFilesMetadataCache;
+use crate::cache::DefaultFilesMetadataCache;
 use datafusion_common::{Result, Statistics};
 use object_store::ObjectMeta;
 use object_store::path::Path;