feat: add extensions boilerplates for Rust and C PostgreSQL extensions

[pyramation]

Summary

Adds a new extensions boilerplate category with workspace and module templates for building PostgreSQL extensions in Rust (using pgrx) and C (using PGXS).

New structure:

• extensions/workspace/ - Makefile-based workspace for managing multiple extensions
• extensions/rust/ - Rust extension module using pgrx 0.12
• extensions/c/ - C extension module using standard PGXS build system

Each module includes build setup, README, gitignore, and a simple "hello" function as a starting point.

Review & Testing Checklist for Human

• Test placeholder substitution - The function names use hello_____moduleName____ (5 underscores) which may conflict with the ____name____ pattern. Scaffold a test project to verify placeholders resolve correctly.
• Verify pgrx version - Cargo.toml uses pgrx 0.12. Confirm this is the appropriate version and that pg13-pg17 feature flags are correct for this version.
• CI workflow completeness - The workspace CI installs PostgreSQL dev headers but does NOT install Rust toolchain. If Rust extensions are expected to build in CI, this needs to be added.
• Test C extension expected output - The expected/____moduleName____.out file has hardcoded column widths that may not match actual psql output formatting.

Recommended test plan:

1. Use pgpm to scaffold a new workspace using extensions/workspace
2. Scaffold both a Rust and C module into it
3. Verify make build and make test work for both extension types

Notes

Requested by: Dan Lynch (@pyramation)
Devin session: https://app.devin.ai/sessions/a5cb8d91114445c982dff9551d9c6e68

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[pyramation]

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