Change GetValue to GetValues for Providers to allow batch retrieval

[doodlesbykumbi]

Succeeds #1344 (Cobertura got in the way)

GetValues works exactly the same as GetValue, however GetValues gives providers the ability to implement arbitrary batch retrieval mechanisms.

What does this PR do?

• What's changed? Why were these changes made?
• How should the reviewer approach this PR, especially if manual tests are required?
• Are there relevant screenshots you can add to the PR description?

What ticket does this PR close?

Connected to #[relevant GitHub issues, eg 76]

Checklists

Change log

• The CHANGELOG has been updated, or
• This PR does not include user-facing changes and doesn't require a CHANGELOG update

Test coverage

• This PR includes new unit and integration tests to go with the code changes, or
• The changes in this PR do not require tests

Documentation

• This PR does not require updating any documentation, or
• Docs (e.g. READMEs) were updated in this PR, and/or there is a follow-on issue to update docs

(For releases only) Manual tests

• Manually tested Keychain provider
• Manually run Kubernetes-Conjur demo with a local Secretless Broker image build of your branch

[codeclimate]

Code Climate has analyzed commit 185bf53 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 90.9% (50% is the threshold).

This pull request will bring the total coverage in the repository to 53.0% (2.7% change).

View more on Code Climate.

[sgnn7]

@doodlesbykumbi LGTM!