Add get upload URL method

[arkaitzgarro]

Proposed changes

Add WeTransfer\File::getUploadUrl method to allow file uploads directly from the clients.

Types of changes

• [ ] Docs (missing or updated docs)
• [ ] Bugfix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Improvement / maintenance (removing technical debt, performance, tooling, etc...)

Checklist

• [x] I have read the CONTRIBUTING doc
• [x] I have added necessary documentation (if appropriate)
• [x] Code changes are covered by unit tests.
• [ ] Any dependent changes have been merged and published in downstream modules

[coveralls]

Pull Request Test Coverage Report for Build 58

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+1.6%) to 97.479%

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Totals
Change from base Build 54:	1.6%
Covered Lines:	116
Relevant Lines:	119

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💛 - Coveralls