Abstract the shared file-saving code between .screenshot() and .pdf()

[adieuadieu]

Following up #84 (comment), we need to abstract the shared code between .screenshot() and .pdf() for saving files to local disk or S3. The abstraction should also incorporate the ability to specify S3 key or local disk path (#83)

There might also be overlap/abstraction possible for/in preparation of #48.

Acceptance Criteria

• Can configure where the resulting data is saved, either by S3 key, or local disk path prefix (Modify path of screenshot/pdf #83)
• Should abstract file saving with Save images or links on element #48 in mind
• makes use of `os.tmpdir() for temp directory (done in Extend screenshot with specific file path in local & element by selector #137, Fix: Generating screenshots/pdf using os.tmpdir() for Windows #172)
• should use Buffer.from() as new Buffer() is deprecated API in newer Node.js versions

[elisherer]

I answered some of the requirements with #137