Introduce certificate pool structure and remove multiple encode/decode process

[arsenalzp]

This PR introduce certificate pool structure which is mainly used to hold Certificates. I want to decouple PEM operations from Certificates pool ones. Deduplication option for certificate pool was also introduced - deduplication process can be optionally disabled. It is related to #305

[cert-manager-prow[bot]]

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[cert-manager-prow[bot]]

Hi @arsenalzp. Thanks for your PR.

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[erikgb]

/ok-to-test

[arsenalzp]

Let me fix typos!