[feature](function) add uniq_theta theta-sketch distinct-count aggregate

[lzydmxy]

Add uniq_theta, a theta-sketch based approximate distinct-count aggregate returning BIGINT, ported from ClickHouse uniqTheta and backed by the Apache DataSketches library (contrib/datasketches-cpp submodule added in #63143). The aggregate reuses the generic agg_state combinator framework so uniq_theta_state / uniq_theta_merge / uniq_theta_union are auto-derived with no extra wiring. Includes BE gtest, FE unit test, and regression suites.

What problem does this PR solve?

Issue Number: close #66240

Related PR: #63143

Problem Summary:
Doris only has HyperLogLog-based approx_count_distinct / ndv, which supports
union but not clean intersection or difference on the serialized state.

This PR adds uniq_theta, an approximate distinct-count aggregate backed by
the Apache DataSketches Theta Sketch (ported from ClickHouse uniqTheta). Theta
sketches add capabilities HLL lacks: set operations (union / intersect /
difference) on the serialized sketch, precompute-once/query-many via mergeable
agg_state, and cross-engine-compatible sketch bytes. Relative error is ~3.125%
(95% confidence, default 4096 nominal entries), exact for small cardinalities.

The contrib/datasketches-cpp submodule and its build wiring were already added
by #63143, so this PR only adds the aggregate itself.

Release note

Add aggregate function uniq_theta, a Theta-sketch (Apache DataSketches) based
approximate distinct-count function, together with its uniq_theta_state /
uniq_theta_merge / uniq_theta_union agg_state combinators.

Check List (For Author)

• Test

  • Regression test
  • Unit Test
  • Manual test (add detailed scripts or steps below)
  • No need to test or manual test. Explain why:
    • This is a refactor/code format and no logic has been changed.
    • Previous test can cover this change.
    • No code files have been changed.
    • Other reason

• Behavior changed:

  • No.
  • Yes.

• Does this need documentation?

  • No.
  • Yes. add docs for aggregation function uniq_theta doris-website#4019

Check List (For Reviewer who merge this PR)

• Confirm the release note
• Confirm test cases
• Confirm document
• Add branch pick label

[hello-stephen]

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