Allow templated legend via attribute keys and values from query results

[heidmotron]

What does this PR do?

Adds a legend formatter so folks can add custom labels that are derived from the data source.

How does this impact users?

If a user doesn't use a query name, than there is no change.
If a user adds a query name with a label name reference from the query results, like{{service}}, then the legend for the that value is interpolated with the label value.

What needed to change in the code and why?

A few changes were made to the Time Series Value fields:

• Labels are appended to the field it self. This will use default formatting and removes the need for our createFieldName function.
• Name is changed to use the TIME_SERIES_VALUE_FIELD which has logic to remove the name if there is no displayNameFromDS set and use default label formatting.
• Query Name allows for variable names in simple mustache like syntax {{var1}} which is similar to the Prometheus / Loki datasource
• If an attribute value doesn't exist in the results set, then the value will be set to <undefined>

How did you test this?

New unit tests and manually.

[MisterSquishy]

i dont have a ton of context here, so worth waiting for @DHedgecock if you need a thorough review. but this all looks great to me!

[DHedgecock]

🚀 Let's go