feat(inputs.x509_cert): Add tag for certificate type-classification

[srebhan]

• Updated associated README.md.
• Wrote appropriate unit tests.
• Pull request title or commits are in conventional commit format

resolves #7012

This PR classifies the type of a certificate into leaf, intermediate and root certificates. The information is added as type tag.

[powersj]

Thank you for the helpful inline comments. Do we have test coverage for these new paths already?

[srebhan]

@powersj the problem with tests using a certificate from file is that those might expire... Let me think about a way...

[powersj]

the problem with tests using a certificate from file is that those might expire...

Right, we do have some have some test certs in testutil/pki. I am cool with adding some certs there as long as you document how they are created.

[powersj]

One test question. As soon as tests pass, ship it

[telegraf-tiger]

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