Jsonnet: Add Grafana Enterprise Logs library

[jdbaldry]

What this PR does / why we need it:
This PR adds a Grafana Enterprise Logs Jsonnet library that extends the OSS Loki library.

Special notes for your reviewer:

• This PR includes rudimentary testing that can be performed with $ make test, this essentially only proves that Tanka is able to evaluate the Jsonnet.
• This has been evaluated against a working dev cluster and produces minimal (and desirable) diffs over what is currently deployed internally.
• Documentation is separate and will be released in the GEL setup docs
• There are some TODOs noted at the top of the library file. These represent task that I would ideally like to contribute to the OSS Loki chart and inherit.

Checklist

• Documentation added
• Tests updated

[jdbaldry]

@pull-request-size pull-request-size bot added the size/XL label 7 minutes ago

Nice!

[dannykopping]

LGTM 👍