Support accessing S3 objects directly via GUI

[paulmueller]

Currently, accessing data on S3 is only possible via basins and by installing via pip and setting S3 credentials in the environment as per https://dclab.readthedocs.io/en/stable/sec_av_s3.html#private-data

• add dclab[s3] extra to the dependency list (installs boto3)
• a prerequisite for the next step (because we can work with latest QtCreator version) would be ref: migrate from PyQt5 to PyQt6 #165
• add an option in the settings to set S3 access keys for (initially just) one endpoint