feat(init): add FLINK preset with Flink-flavored function registration

[ggjh-159]

related issue: #51

Description

Adds a FLINK preset so Flink applications get Flink-flavored function registrations instead of inheriting Spark semantics.

What's in this PR

• Config.h — enum Preset { SPARK = 0, FLINK = 1 }.
• Config.cc — custom presetToStr/presetFromString (the default folly::tryTo<Preset> rejects the enum and fails at runtime with "Value 'FLINK' cannot be converted to type velox4j::Preset").
• Init.cc:
  • Extracts registerConnectors() out of initForSpark() — the 9 connector serde/registration blocks (fuzzer / hive / kafka / pulsar / filesystem / external-stream / nexmark / print / from-elements) now live in their own function.
  • Removes the regexp_extract override inside initForSpark() (reverting PR Fix REGEXP_EXTRACT to return NULL instead of empty string on no match #31's global override).
  • Adds initForFlink() that calls initForSpark() + functions::flinksql::registerFunctions(). The reuse is intentional: velox/functions/flinksql/ does not yet ship a full scalar/aggregate/window function set, so Flink still rides on sparksql as the baseline; the flinksql entry only overrides dialect-specific semantics(feat: Add Flink-flavored REGEXP_EXTRACT in functions/flinksql velox#68).
  • Switch dispatches on the configured preset.
• Preset.java — KEY, SPARK, FLINK constants for Velox4j.configure(...).

Testing

mvn test -Dtest=RegexpExtractTest — 3/3 PASS under the FLINK preset (no-match returns NULL).