Add new blog post about the signer API

[jku]

This gives some example application code for the new securesystemslib.signer API.

[jku]

I think the easiest way to review is to read https://github.com/jku/python-tuf/blob/securesystemslib-blog/docs/_posts/2023-01-24-securesystemslib-signer-api.md -- only the header block is unhandled, otherwise it should look like the published post

[coveralls]

Pull Request Test Coverage Report for Build 4013939676

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 98.139%

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Totals
Change from base Build 4013469727:	0.0%
Covered Lines:	1344
Relevant Lines:	1361

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💛 - Coveralls

[jku]

CC @VickyMerzOwn: you mentioned you might be interested in reviewing

[lukpueh]

Super cool post! I left two suggestions inline (feel free to ignore). Did you consider adding a third code example that showcases dynamic dispatch for public key and verification?

[jku]

Amended the commit with the two minor suggestions: both clarify that private key URIs are constructed while public keys are imported from the underlying signing system.

[jku]

Did you consider adding a third code example that showcases dynamic dispatch for public key and verification?

I did... Since the application code looks exactly what it looked like before, it's not that interesting.

We of course know that the dispatch will enable GPG or sigstore keys but that's not really visible right now