Add support streamer data transfers

[ekouts]

Would be better to merge after eth-cscs/firecrest-v2#169, so that we also pin the version to the latest firecrest-streamer

[Copilot]

Pull request overview

This PR adds support for streamer-based data transfers as an alternative to S3, enabling more flexible file transfer options. It also modernizes the Python version requirements by dropping Python 3.7 support and adding support for Python 3.10-3.12.

Key changes:

• Introduces streamer transfer method alongside existing S3 method for both uploads and downloads
• Adds firecrest-streamer>=0.00.21 dependency and corresponding mypy configuration
• Updates Python version support from 3.7+ to 3.8+ with explicit support through 3.12

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 10 comments.

Show a summary per file

File	Description
pyproject.toml	Updates Python version classifiers (removes 3.7, adds 3.10-3.12), sets minimum to 3.8, and adds firecrest-streamer dependency
firecrest/v2/_sync/Client.py	Adds ExternalTransfer base class, implements streamer upload/download methods, updates transfer_method parameter validation, and adds wait_for_streamer_job_to_listen logic
firecrest/v2/_async/Client.py	Mirrors sync client changes with async equivalents for streamer transfers and job waiting
firecrest/utilities.py	Adds sched_state_running helper function to detect running job states
.mypy.ini	Adds streamer module to ignore_missing_imports configuration
.github/workflows/testing.yml	Updates test matrix to remove Python 3.7

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