Upgrade RestSharp from 106.15.0 to 114.0.0

[weppos]

RestSharp 106.15.0 was released in 2021. We deferred the upgrade for so long that the dependency has since undergone a full architectural rewrite. 😓

For instance, v107 switched to HttpClient, removed the IRestClient / IRestRequest / IRestResponse interfaces, made client options immutable post-construction, renamed the Method enum to PascalCase, removed JsonObject, etc.

This PR performs that migration in one go, keeping the public DNSimple client surface unchanged:

• Rewrote RestClientWrapper to own RestClientOptions and lazily (re)build the underlying RestClient whenever BaseUrl, UserAgent, or the authenticator change — required because v114 makes those immutable after construction.
• Replaced IRestClient / IRestRequest / IRestResponse with the concrete types throughout HttpService, ServiceBase, RegistrarDelegation, and all mocks/tests. Response.Headers is now IReadOnlyCollection<HeaderParameter>.
• Migrated Credentials to the new IAuthenticator contract (and the Encoding-aware HttpBasicAuthenticator constructor).
• Updated RequestBuilder for the v114 RestRequest constructor, and switched serialized-JSON payloads to AddStringBody(..., DataFormat.Json) to avoid double-encoding under the new AddJsonBody(string) overload.
• Replaced JsonObject (removed in v107) in DomainsPushes with a plain Dictionary<string, object>.
• Updated the Method enum casing (GET → Get, etc.) across all services and tests.
• Reworked HttpTest.InvalidRequest to exercise HttpService.HandleExceptions directly (exposed as internal with InternalsVisibleTo dnsimple-test), since the previous test mocked interfaces that no longer exist.

Closes #223.

📋 Deployment Pre/Post tasks

N/A

Deployment Verification

• CI passes on this PR (build + 243 existing tests).
• The next release of the DNSimple NuGet package installs cleanly into a consumer project and authenticates against the sandbox with an OAuth2 token.

[san983]

Looks good, tested locally and went thru all the diff.

Noticed 2 instances of this warning

'DomainsService.InitiatePush(long, string, string)' is obsolete: 'Use InitiatePushWithIdentifier instead'

But those can be fixed in a new PR