imgtool: Add tests for imgtool commands part 1

[rustammendel]

Add various tests and improvements for imgtool commands

• Add tests for sign command
• Add tests for dumpinfo command
• Add tests for verify command
• Add more tests for key generation and derivation
• Add pre-generated images and keys for testing
• Move key unittests to tests/keys directory
• Improve file input and error handling for some commands
• Improve hex format support across commands
• Refactoring and linting

[davidvincze]

Please also update the cryptography dependency in scripts/setup.py:17

[rustammendel]

Commit ded02c9 fixes issue 1846 and might be breaking for some users that use integer type custom TLVs.

[github-actions[bot]]

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[timonburkard]

@rustammendel thanks for your effort!

This PR is open for a long time, what do you think, would it make sense to create a separate PR for the fix in imgtool dumpinfo for HEX files? (44e14a31f660cbf68a70ca508c2624d361e12675)

From my understanding this resolves #1976

This one is important for us. Like this I hope we'll be able to speed it up?

@kszromek-nordic