Add regexp filter for artifact name

[EnricoMi]

Currently, either one artifact (by name) or all artifacts can be downloaded.

This adds option filter, which is used as a regular expression to match the artifact names. Only those artifacts where the name matches the regular expression are downloaded.

[dawidd6]

• name and filter inputs conflict with each other, what happens is someone specifies both?
• you need to add the input to action.yml too
• some CI job to test this new input would be nice
• I would lean towards a slightly different way, make filter input be a boolean and treat name input as regex if filter: true (defaulting to false for backward compat)

[dawidd6]

Nice work, thanks!

[EnricoMi]

Works, thanks for merging so quickly.