Add default environment prefix to allow environments and dataset with same name in rules file

[MikeGoldsmith]

Is your feature request related to a problem? Please describe.
When configuring refinery rules, you cannot specify both an environment and dataset with the same name. This is possible to configure in Honeycomb - eg a legacy production dataset and a production environment.

Describe the solution you'd like
Add a legacy dataset prefix config setting that differentiate between environments and datasets.

The default would be empty, so no prefix is used.

eg
config.toml

...
LegacyDatasetPrefix = "legacy"
...

rules.toml

# environment
[production]
...

# dataset
[legacy.production]
...

Describe alternatives you've considered

Additional context

[seh]

Rather than "legacy" being the default prefix, consider the word "dataset" as an alternative.

[MikeGoldsmith]

@seh I've created the PR to allow a dataset prefix. Feel free to take a look 👍🏻