feat(c/db2): add initial connection-only Db2 driver

[nishantchandraa]

Summary

This PR introduces an initial C/C++ ADBC driver for IBM Db2, intentionally scoped to connection management only so maintainers can review a small, focused baseline.

Included in this PR

• New Db2 driver skeleton integrated into C/C++ build and packaging wiring
  • CMake option and subdirectory integration
  • Meson integration
  • Driver config and pkg-config metadata
• DB2 database/connection lifecycle implementation
  • Database init/release (SQLHENV management)
  • Connection init/release (SQLHDBC allocation, connect, disconnect)
  • Default autocommit behavior on connection init
• Connection option handling
  • uri support
  • per-field options: database, hostname, port, uid, pwd
  • standard username / password synonyms
  • explicit precedence: uri overrides conflicting per-field options
• Connection-level validation/error behavior
  • malformed port rejected (1-65535 required)
  • empty UID/PWD rejected
  • SQLSTATE-to-ADBC status mapping for connection/auth failures
• Minimal connection-focused tests and docs
  • lifecycle and concurrent-connection flow
  • missing/unknown option handling
  • invalid credentials handling
  • unreachable endpoint mapping
  • option precedence and validation behavior
  • DB2 driver docs + index inclusion

Explicitly out of scope

• Statement execution
• Result fetching/cursor handling
• Metadata APIs (GetInfo, GetObjects, schemas/catalog)
• Transaction APIs beyond default connection behavior
• Bulk ingest and advanced type mapping
• Performance optimizations

Validation

Tested against a live Db2 instance on localhost:50000:

• adbc-driver-db2-test: 15/15 passed
• ctest -L driver-db2: pass

Coverage includes:

• database/connection init+release
• autocommit default and concurrent connection flow
• invalid credentials
• malformed port
• empty UID/PWD
• unreachable host/port error mapping
• uri precedence over per-field options

Notes for reviewers

This PR is intentionally narrow to establish a clean, reviewable baseline for DB2 support in ADBC. Follow-up PRs will add statement execution, metadata, transaction functionality, and broader feature parity incrementally.