[Bugfix] Speed up process shutdown

[fhibf]

Description

We noticed the sporadic occurrence of ObjectDisposedReferenceExceptions in our service.
That can be caused by the BackgroundLoop keeping the process longer than the required running.

Adding new exception of type FhirServiceUnavailableException, to indicate the service is no more able to handle requests as it's shutting down.

Related issues

AB#118543

Testing

Describe how this change was tested.

FHIR Team Checklist

• Update the title of the PR to be succinct and less than 65 characters
• Add a milestone to the PR for the sprint that it is merged (i.e. add S47)
• Tag the PR with the type of update: Bug, Build, Dependencies, Enhancement, New-Feature or Documentation
• Tag the PR with Open source, Azure API for FHIR (CosmosDB or common code) or Azure Healthcare APIs (SQL or common code) to specify where this change is intended to be released.
• Tag the PR with Schema Version backward compatible or Schema Version backward incompatible or Schema Version unchanged if this adds or updates Sql script which is/is not backward compatible with the code.
• CI is green before merge
• Review squash-merge requirements

Semver Change (docs)

Patch|Skip|Feature|Breaking (reason)