Flux query builder fluent API

Nobody has claimed this yet.

Currently, the queries have to be built manually in a string. It would be great to have a fluent API to build a query.

I know you guys are working on another library to query by using LINQ, but that requires having the data modeled and annotated in C# classes, which is not always possible due to the nature of dynamic data and metrics.

I'm proposing an API that can do things like the following:

```csharp
var query = influxDb.GetQueryBuilder()
.From("bucket")
.Filter(r => r["_measurement"] == "cpu")
.Filter(r => r["_field"] == "temperature")
.Filter(r => r["some-tag"] == "some-value")
.AggregateWindow(every: TimeSpan.FromSeconds(10), fn: InfluxDb.Functions.Mean, createEmpty: false)
.Yield("mean")
.ToString();
```

This way we could be writing queries in a .NET idiomatic way and ensuring the resulting strings are valid Flux queries.

No files, tests, or entry points are named. Start by surveying the existing manual Flux query construction and the separate LINQ-based query library mentioned in the issue, then determine the scope and API shape for dynamic data and metrics; done means an agreed fluent API can produce valid Flux queries like the example.

Written by the indexing model from the issue text.