Add Unavailable result status

[DorianGreen]

When making calls to downstream services or databases transient errors could occur that may get resolved if the operation is retried.

This PR adds an Unavailable result status and a mapping to a 503 Service Unavailable http result.

Usage scenarios:

1. A service adds a resiliency layer (using Polly for example) to handle retries.
   instead of handling various exception or error types in the shared resiliency layer, the developer can handle these cases closer to where they occur and return Result.Unavailable. the resiliency layer can now be made generic, and error agnostic, and retry based on the result status.

2. while making a call to a downstream service, we get some transient error (database unavailable). the service layer can handle the error and return Result.Unavailable and the result will be converted to a 503 http response instead of a default 500 or 422.
   Callers of our API get a better experience and can add their own resiliency layers if needed.

[github-actions]

Package	Line Rate	Branch Rate	Complexity	Health
Ardalis.Result	0%	0%	79	❌
Ardalis.Result.AspNetCore	38%	45%	159	❌
Summary	23% (120 / 515)	40% (59 / 146)	238	❌

[ardalis]

Thanks!