[8.4](backport #848) Reload downloader client on config change

[michalpristas]

Backport of #848 and #776 as prerequisite

What does this PR do?

Downloaders have own instance of client which is used to download artifacts.
This client is created when constructing a downloader instance and exists for the time being.
When configuration changes for artifacts.download client should be recreated to reflect new proxy/TLS/timeout settings.

On top of that this also fixes an issue when agent after failed download does not remove corrupted files which leads to retry of verification loop which fails on asc and sha512 not found errors making downloads and upgrades unusable in a future.
This is source of many SDHs which requires manual intervention and cleaning up download directory by user.

Why is it important?

• reflecting changes in a client
• reducing incident count for failed upgrades
• required to properly respond to proxy changes

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• Start Time: 2022-08-09T12:42:22.783+0000

• Duration: 16 min 58 sec

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Skipped	23
Total	6157

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🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	97.333% (73/75)	👍 0.111
Files	68.996% (158/229)	👍 0.978
Classes	69.095% (313/453)	👍 0.995
Methods	53.034% (874/1648)	👍 1.026
Lines	39.208% (9451/24105)	👍 0.875
Conditionals	100.0% (0/0)	💚