logging: allow passing logger to `NewRunner`

Ref code: https://github.com/hashicorp/consul-template/blob/e60eab92c0436e63c73c05060c8c916870acd509/manager/runner.go#L209

Nomad utilizes consul-template as a library and it would be great if we could optionally pass a logger when setting up a new runner. This could avoid some of the intercept overhead from our template runner associated to logging which is currently required to get CT logs showing up within the Nomad client logging.

Related: https://github.com/hashicorp/consul-template/issues/1522

Start at manager/runner.go around NewRunner, using the referenced revision, and read related issue 1522 for context on Nomad's logging integration. Determine how NewRunner can optionally accept a logger without requiring logging interception; done means library users can provide one while existing callers continue to work.

Written by the indexing model from the issue text.