refactor(rust): introduce a new "transfer" module

[imobachgs]

• Introduce a new transfer module containing an API to download files.
• The idea is to extend this API in the future to support other URL
  types.
• This module replaces the old "download" function.

[coveralls]

Pull Request Test Coverage Report for Build 11255280668

Details

• 0 of 9 (0.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.003%) to 70.962%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
rust/agama-cli/src/lib.rs	0	1	0.0%
rust/agama-cli/src/profile.rs	0	1	0.0%
rust/agama-lib/src/transfer.rs	0	7	0.0%

Totals
Change from base Build 11252810563:	0.003%
Covered Lines:	16092
Relevant Lines:	22677

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💛 - Coveralls