Add aggregate expressions to Continuous Aggregate

[fabriziomello]

Implements add_continuous_aggregate_column() SQL function that allows adding new aggregate expressions to existing Continuous Aggregates without recreating them.

The function updates all internal CAgg structures:

• Materialization hypertable (adds column for storing computed values)
• Partial and Direct views (adds aggregate computation)
• User view (adds column reference or aggregate for real-time mode)

Usage examples:

-- Add aggregate with explicit alias
SELECT add_continuous_aggregate_column('my_cagg', 'sum(value) AS total');

-- Add aggregate without alias (uses function name)
SELECT add_continuous_aggregate_column('my_cagg', 'avg(temperature)');

-- Skip if column already exists
SELECT add_continuous_aggregate_column('my_cagg', 'max(value) AS peak', true);

Limitations:

• Cannot add aggregates to CAggs with compression enabled
• Only single aggregate expressions allowed per call
• Column name must not already exist in the CAgg

[codecov]

Codecov Report

❌ Patch coverage is 86.27451% with 35 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
tsl/src/continuous_aggs/alter.c	85.83%	11 Missing and 22 partials ⚠️
tsl/src/continuous_aggs/common.c	89.47%	1 Missing and 1 partial ⚠️

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