Fix issue with debug output masking if it contains special symbols

[alexander-smolyakov]

Issue description:
Currently, the secret masker doesn't mask debug output of the task.setvariable command if it contains specials symbols in the variable value (e.g. %).

Example:

Before percent decode
secretName = text_percents_azp25
secretValue = {"first":{"second":{"third":{"password":"%AZP25%AZP25%AZP25%AZP25%AZP25%AZP25%AZP25%AZP25%AZP25%AZP25%AZP25%AZP25%AZP25%AZP25"}}}}
--------------------
After percent decode
secretName = text_percents_azp25
secretValue = {"first":{"second":{"third":{"password":"%AZP25AZP25%AZP25AZP25%AZP25AZP25%AZP25AZP25%AZP25AZP25%AZP25AZP25%AZP25AZP25%AZP25AZP25%AZP25AZP25%AZP25AZP25%AZP25AZP25%AZP25AZP25%AZP25AZP25%AZP25AZP25"}}}}
##[debug]set text_percents_azp25=********
##[debug]Processed: ##vso[task.setvariable variable=text_percents_azp25;issecret=true;]{"first":{"second":{"third":{"password":"%AZP25AZP25%AZP25AZP25%AZP25AZP25%AZP25AZP25%AZP25AZP25%AZP25AZP25%AZP25AZP25%AZP25AZP25%AZP25AZP25%AZP25AZP25%AZP25AZP25%AZP25AZP25%AZP25AZP25%AZP25AZP25"}}}}
--------------------------------------

Fix description:
It seems the issue is related to the fact that we adding only unescaped command strings to the masker dictionary. To fix this problem we should add to the secret masker dictionary both escaped and unescaped strings.

Changes

• Extracted logic for escaping and unescaping special symbols to CommandStringConvertor class
• Added escape data of ##vso commands to the secret masker to resolve the problem with debug output masking
• Removed Agent.Sdk.Knob from the Command class as it is not used in the code.

Documentation changes required: No

Added unit tests: No

Attached related issue:

• #1326

[mjroghelia]

Looks ok to me. Please hold off on merging until @damccorm has reviewed.

[damccorm]

Looks good to me - thanks for doing this!

[EzzhevNikita]

Tested these changes manually seems like everything work as expected

[DaniilShmelev]

LGTM, thanks!