cloud-auth: implement gcp(user-managed-service-account())

[alltilla]

This authentication method can be used on VMs in GCP to use the linked service.

Example minimal config, which tries to use the "default" service account:

cloud-auth(
  gcp(
    user-managed-service-account()
  )
)

Full config:

cloud-auth(
  gcp(
    user-managed-service-account(
      name("alltilla@syslog-ng-test-project.iam.gserviceaccount.com")
      metadata-url("my-custom-metadata-server:8080")
    )
  )
)

This authentication method is extremely useful with syslog-ng's google-pubsub() destination,
when it is running on VMs in GCP, for example:

destination {
  google-pubsub(
    project("syslog-ng-test-project")
    topic("syslog-ng-test-topic")
    auth(user-managed-service-account())
  );
};

For more info about this GCP authentication method, see:

• https://cloud.google.com/compute/docs/access/authenticate-workloads#curl
• https://cloud.google.com/compute/docs/access/create-enable-service-accounts-for-instances

[github-actions]

This Pull Request introduces config grammar changes

syslog-ng/2bbd52018ef8e991796f243337806eb7259d9730 -> alltilla/cloud-auth-gcp-user-managed-service-account

--- a/destination
+++ b/destination

 http(
     cloud-auth(
         gcp(
+            user-managed-service-account(
+                <empty>
+                metadata-url(<string>)
+                name(<string>)
+            )
         )
     )
 )

[bazsi]

The Mac build fails, but strangely it uses ivykis from the system not from the submodule, where some of the fixes would be.

I am also puzzled why it does not find the picojson symbol, which it finds on all three other combinations (automake with both clang/gcc, and cmake with clang works, cmake with gcc does not).

That symbol is coming from a header only lib which has the symbol on my system.

[alltilla]

According to http://demangler.com/

The missing symbol __ZN8picojson5value3getIlEERT_v is _long& picojson::value::get<long>().

The int64_t support is not always enabled in picojson: https://github.com/kazuho/picojson?tab=readme-ov-file#experimental-support-for-int64_t, probably that macOS build is not enabling it by default, but others do.

I'll just get a double and round it.

[bazsi]

This is not performance critical so I am fine with using double here.

[ThomasDevoogdt]

The configure.ac should not longer automaticially enable cloud-auth, similar to enable_http, a check for curl should be done here.