MAST: Enable cloud dataset by default

[snbianco]

Current Behavior

• Access to cloud dataset is not enabled by default.
• Users have to explicitly run the enable_cloud_dataset function before they use any cloud-related functions.

Proposed Behavior

• The cloud dataset is enabled by default, so users do not have to explicitly call enable_cloud_dataset.
• To disable the cloud dataset, users can alter a config value or call the disable_cloud_dataset function.

Advantages

• Faster downloads (in some cases)
• Removes a hidden step that users may not even know is available to them.
• More in line with the direction that MAST is heading re. Roman and other large datasets
• More convenient for users in cloud platforms
• Fewer download failures
• Reduce operational load on MAST servers

What Changes for Users?

• No longer have to call enable_cloud_dataset explicitly
• Cloud downloads will be preferred automatically.
• Files in the cloud will be pulled from there instead of MAST servers.
• This is just default behavior and can be changed.
• Cloud dataset will NOT be enabled if the user does not have the prerequisite packages (boto3, botocore)
• Users can change whether the cloud dataset is automatically enabled with a configuration variable

Other Things to Note

• Cloud access is instantiated lazily only when a relevant method is called (download_file, download_products, get_cloud_uri(s))
• Fixes a bug introduced in Get cloud missions dynamically and fix cloud download workflow #3488 where botocore, an optional package, is imported without a guard. Now, both boto3 and botocore are imported with try blocks and ImportError is properly handled.
• Observations internally keeps track of whether the cloud dataset is explicitly enabled or disabled. If not explicitly enabled by the user (they call enable_cloud_dataset), warning messages are not logged when a product cannot be found in the cloud and the download falls back to on-prem. This is to avoid clogging up the console and introducing warning messages where there were none before.

Close #3546

[codecov]

Codecov Report

❌ Patch coverage is 83.92857% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.71%. Comparing base (de342ea) to head (62832b0).
⚠️ Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
astroquery/mast/cloud.py	69.23%	4 Missing ⚠️
astroquery/exceptions.py	0.00%	3 Missing ⚠️
astroquery/mast/observations.py	94.87%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3534   +/-   ##
=======================================
  Coverage   72.71%   72.71%           
=======================================
  Files         219      219           
  Lines       20473    20528   +55     
=======================================
+ Hits        14886    14927   +41     
- Misses       5587     5601   +14     

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