Add standard Jackson JSON media type to admin API

[mgaffigan]

Adds application/mirthapi+json as a supported content-type for the admin API's that serialize or deserialize objects.

• Avoids the round-trip through XML as currently performed by ObjectJSONSerializer/JsonMessageBodyReader
• Matches the OpenAPI spec, allowing auto-generated clients in most languages
• Probably more performant, but not really the point.

application/json keeps the historical behavior - this functionality is opt-in.

Closes #188

[mgaffigan]

Here's a few examples of the implementation:

Login:

POST https://localhost:8443/api/users/_login HTTP/1.1
User-Agent: Fiddler
X-Requested-With: Foo
Host: localhost:8443
Content-Type: application/x-www-form-urlencoded
Accept: application/mirthapi+json
Content-Length: 29

username=admin&password=admin

HTTP/1.1 200 OK
Date: Sat, 18 Oct 2025 19:03:49 GMT
Access-Control-Allow-Origin: *
Access-Control-Allow-Credentials: false
Access-Control-Allow-Methods: GET, POST, DELETE, PUT
Access-Control-Allow-Headers: Content-Type
Content-Security-Policy: frame-ancestors 'none'
X-Frame-Options: DENY
X-Content-Type-Options: nosniff
Strict-Transport-Security: max-age=31536000; includeSubDomains
Set-Cookie: JSESSIONID=node05y5j2mlzbr1hslxmsncseev91.node0; Path=/api; Secure
Expires: Thu, 01 Jan 1970 00:00:00 GMT
Content-Type: application/mirthapi+json
Content-Length: 33

{"status":"SUCCESS","message":""}

Get Current User:

GET https://localhost:8443/api/users/current HTTP/1.1
User-Agent: Fiddler
X-Requested-With: Foo
Host: localhost:8443
Accept: application/mirthapi+json
Cookie: JSESSIONID=node05y5j2mlzbr1hslxmsncseev91.node0

HTTP/1.1 200 OK
Date: Sat, 18 Oct 2025 19:05:02 GMT
Access-Control-Allow-Origin: *
Access-Control-Allow-Credentials: false
Access-Control-Allow-Methods: GET, POST, DELETE, PUT
Access-Control-Allow-Headers: Content-Type
Content-Security-Policy: frame-ancestors 'none'
X-Frame-Options: DENY
X-Content-Type-Options: nosniff
Strict-Transport-Security: max-age=31536000; includeSubDomains
Content-Type: application/mirthapi+json
Content-Length: 107

{"id":1,"username":"admin","lastLogin":"2025-10-18T19:03:49.622+00:00","strikeCount":0,"userConsent":false}

Add user:

POST https://localhost:8443/api/users/ HTTP/1.1
User-Agent: Fiddler
X-Requested-With: Foo
Host: localhost:8443
Content-Type: application/mirthapi+json
Accept: application/mirthapi+json
Cookie: JSESSIONID=node05y5j2mlzbr1hslxmsncseev91.node0
Content-Length: 22

{"username":"example"}

HTTP/1.1 204 No Content
Date: Sat, 18 Oct 2025 19:06:07 GMT
Access-Control-Allow-Origin: *
Access-Control-Allow-Credentials: false
Access-Control-Allow-Methods: GET, POST, DELETE, PUT
Access-Control-Allow-Headers: Content-Type
Content-Security-Policy: frame-ancestors 'none'
X-Frame-Options: DENY
X-Content-Type-Options: nosniff
Strict-Transport-Security: max-age=31536000; includeSubDomains