Add configurable timeouts for file downloads

[sylvesterkaczmarek]

Jira Issue ID

No Jira ID supplied. Fixes #122.

Description

Add optional Client(download_timeout=...) socket limits for file downloads. Accept a scalar or a (connect, read) pair and apply it to direct, bulk, intermediate-file, and OPeNDAP downloads. Default None preserves existing behavior; authentication and job polling are unchanged.

Document inactivity versus total-duration limits, retry behavior, Future exceptions, and partial-file retry handling. This does not add automatic partial-file cleanup or a total-download deadline.

Local Test Steps

macOS arm64, Python 3.12.11:

• pytest --cov=harmony --cov-branch tests: 256 passed, including 17 new timeout cases.
• Real localhost HTTP tests stall both response headers and bodies, verify timeout propagation through Future.result(), and confirm a single-worker executor becomes available again.
• Removing the socket timeout argument makes both stalled-server regressions fail, independently of the new constructor API.
• Ruff checks/formatting and git diff --check passed.

No live Earthdata downloads, multi-gigabyte transfers, or Windows/Linux execution were performed.

PR Acceptance Checklist

• Requested timeout option implemented with backward-compatible defaults.
• Tests added and passing.
• Documentation updated.