Add context to method calls

[k-capehart]

This was originally included here: 43d91e6

I decided to make this its own issue to better analyze the use cases and decide when it felt like a good candidate to be merged.

Original commit message:

This commit adds context.Context support to all Salesforce client methods and internal functions, enabling request cancellation and timeout control, and updates examples and documentation to reflect these changes.

• Inject ctx context.Context parameter into all public and internal methods and helper functions
• Switch to http.NewRequestWithContext in doRequest and propagate ctx through retry/error flows
• Update tests, examples, and README to use t.Context() or context.Background()