Spike: Update HttpClient usage to better support DNS changes on .NET/.NET Core

[nrcventura]

Describe the story, please be clear on scope of the story.

The dotnet agent's current usage of HttpClient for sending data to New Relic mostly follows MIcrosoft's guidance (this analysis is documented in an internal doc). The one thing that was discovered to be missing is setting the PooledConnectionLifetime to an appropriate value (this value is TBD). The agent is already using a single instance of HttpClient.

Acceptance Criteria

1. The agent uses a single instance of HttpClient to send data to New Relic.
2. The agent configures an appropriate value for the PooledConnectionLifetime setting.
3. These changes apply to .net/.net core, and do not apply to .net framework.

Dependencies

N/A

Estimates

t-shirt size: S

[workato-integration]

https://issues.newrelic.com/browse/NR-150024