Add optional baseURL override to Configuration

[josebet1]

Motivation

• Enable overriding the SDK base URL so integration tests and local mocks can target a local or self-hosted endpoint instead of Method's remote environments.
• Prevent accidental calls to production/sandbox during local development by allowing explicit endpoint control.
• Maintain existing environment-driven defaults and validation while following common SaaS SDK patterns (e.g. Stripe) for configurability.

Description

• Added an optional baseURL?: string field to the IConfigurationOpts interface.
• Updated Configuration constructor to use opts.baseURL ?? \\https://${opts.env}.methodfi.com\`` and normalize the value by stripping trailing slashes with baseURL.replace(/\/+$/, '').
• Added URL validation in Configuration._validateConfiguration to throw a clear error for invalid baseURL values.
• Documented the new baseURL option in the README.md usage example showing how to point the SDK at http://localhost:4010 for local mocks.

Testing

• No automated tests were run for this change.

---

Codex Task

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 07d91bacd6

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[chatgpt-codex-connector]

Reject empty baseURL values

If a caller passes baseURL: '' (e.g., from an unset env var), the opts.baseURL check is falsy so validation is skipped, but the constructor still uses the empty string because ?? does not fall back on ''. This results in Configuration.baseURL being empty, and later new URL(${request.baseURL}${request.url}) in src/resource.ts throws for relative-only URLs, breaking all requests at runtime. Consider treating empty/whitespace strings as invalid or falling back to the env default.

Useful? React with 👍 / 👎.