feat(environments): read DevX Core URL from config (DEVX-793)

[smukherjee-godaddy]

Summary

• read devx_core_url from each active environments.toml entry
• preserve <ENV>_DEVX_CORE_URL and DEVX_CORE_URL as higher-precedence runtime overrides
• retain built-in DevX Core URLs for prod and ote, and cover file configuration, precedence, and invalid URLs

Example

[dev]
api_url = "https://api.dev-godaddy.com"
client_id = "..."
devx_core_url = "https://api.developer.commerce.dev-godaddy.com"

Validation

• cargo check
• cargo clippy -- -D warnings
• cargo test (552 passed)
• cargo fmt --check

https://godaddy-corp.atlassian.net/browse/DEVX-793

[jpage-godaddy]

It'd be simpler if you added the config value to BUILTIN_ENVS and extended BaseEnvConfig; then you don't need a separate environment-to-values lookup table or this function. The default function is mainly for if you needed to compute a default; for pre-populating built-in environments, we already have a mechanism for that.