Certhandler secrets vault (#87)

* vault

* vault implementation draft

* linting

* add doctring

* more linting here and there

* fix static-lib check

* increase LIBPATCH

* linting here, linting there, linting everywhere

* lint and static fixes

* fmt

* Apply suggestions from code review

Co-authored-by: Leon <[email protected]>

* local changes

* remove raise RuntimeError since the generation is in `private_key` property

* pr comments

* fmt

* fix ImportError message

* new itests added

* fmt...

* add TODO comments

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Co-authored-by: Jose C. Massón <[email protected]>
Co-authored-by: Leon <[email protected]>