Retrieve NuGet API Key via REST

Currently the NuGet API Key can only be retrieved manually via the User menu.

We have automated pipelines that provisioning user's/team's tools, and having to manually retrieve the NuGet API Key throws a wrench into an otherwise fully-automated process.

Example

Similar to [Nexus IQ's User Token REST API|https://help.sonatype.com/iqserver/automating/rest-apis/user-token-rest-api---v2] (albeit keeping the conventions of NXRM's API), there could be an endpoint:

POST /v1/security/nuget-apikey/currentUser
{
  "apiKey":"0569d499-ffc2-452d-bce1-8ebfa98f5c23"
}

 The design of this would likely be influenced by whatever comes of NEXUS-12653.

This is similar to NEXUS-12653 and NEXUS-21354.