feat(cli): add --env and --env-file for local runtime env vars

[mercael91]

Summary

Adds two new CLI options to the functions-framework entry point for passing runtime environment variables during local testing.

Changes

• --env KEY=VALUE (repeatable): Set individual runtime environment variables before the function module is loaded.
• --env-file PATH: Load runtime environment variables from a .env file. Supports comments (#), blank lines, and quoted values.

Behavior

• Values from --env-file are applied first, then any --env flags override them.
• This allows developers to maintain a base .env file while selectively overriding specific variables for different test scenarios.

.env file support

• Comment lines (starting with #)
• Blank lines
• Double-quoted and single-quoted values
• Values with leading/trailing whitespace (trimmed)

Usage examples

# Inline env vars
functions-framework --target=my_func --env DB_HOST=localhost --env DB_PORT=5432

# From a .env file
functions-framework --target=my_func --env-file .env.local

# File + overrides
functions-framework --target=my_func --env-file .env --env DEBUG=true

Tests

6 new tests covering:

• --env sets environment variables
• --env-file loads from file (with comments, quotes, whitespace)
• --env overrides --env-file values
• Invalid --env format error
• Missing --env-file error
• Invalid --env-file line format error

All 18 tests pass.

Fixes #215

CLA: signed