Provide meta function to query column index sizes

Title: Provide meta function to query column index sizes

Description: StarRocks already provides functions like column_size and column_compressed_size to inspect the storage size of a column. However, there is currently no way to check the index size of a column. Indexes can consume a significant portion of storage, and visibility is important for diagnostics and optimization.

Proposed Feature: Introduce a meta function for index sizes, aligned with the style of existing column size functions.

Function Signature (proposal):

-- Must be used in META scan context (same style as column_size/column_compressed_size)
index_size(column_ref [, 'BITMAP' | 'INVERTED' | 'ZONEMAP' | 'BLOOM' | 'ALL']) -> BIGINT

Behavior:

• Can only be used in FROM <table> [_META_] context.
• Returns the size in bytes of the specified column’s index(es).
• If the second argument is omitted or 'ALL', returns the sum of all index sizes for that column.
• If a specific index type is provided (e.g., 'BITMAP'), only that type’s size is returned.
• Semantics are consistent with column_size(col) / column_compressed_size(col) (aggregation from segment metadata).

Examples:

-- Total size of all indexes on the column
SELECT index_size(lo_orderdate)
FROM ssb.lineorder [_META_];

-- Size of a specific index type
SELECT index_size(lo_orderdate, 'BITMAP')
FROM ssb.lineorder [_META_];

-- With partition/tablet hint (if supported)
SELECT index_size(lo_orderdate, 'INVERTED')
FROM ssb.lineorder PARTITION(p202409) [_META_];

Use Cases:

1. Diagnostics: Inspect index overhead for a single column.
2. Optimization: Compare different index types in terms of storage cost.
3. Capacity Planning: Estimate overall index storage contribution at the column level.

Impact: Provides a consistent and simple way to measure index sizes, aligned with the existing meta functions for column sizes.

TODO (Future Work): Consider exposing the same index size information in information_schema for easier programmatic access and integration with monitoring tools.