Add base64 encode / decode custom template functions

[szh]

Desired Outcome

We would like to add support for two Golang template functions to allow users of Push to File custom templates to have the Secrets Provider perform base64 encoding and base64 decoding (respectively) on secrets retrieved from Conjur before rendering the secret values to a secret file.

Implemented Changes

Added support for b64enc and b64dec template functions.
Implemented using the standard Golang encoding/base64 package.

Connected Issue/Story

CyberArk internal issue link: ONYX-14094

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[diverdane]

That was quick! I like the separate file for custom template functions. LGTM!!!

[codeclimate]

Code Climate has analyzed commit e3333d8 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (50% is the threshold).

This pull request will bring the total coverage in the repository to 91.2% (0.1% change).

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