[Alerting] change eventLog ILM requests to absolute URLs

resolves elastic#68265

This changes the ILM requests made by the eventLog from relative to absolute
URLs.  These requests test the existence of and create ILM policies, and are
made with a cluster client using `transport.request`.  Relative URLs work fine
locally and in CI, however do not work on the cloud.

x-pack/plugins/event_log/server/es/cluster_client_adapter.test.ts

@@ -51,7 +51,7 @@ describe('doesIlmPolicyExist', () => {
     await clusterClientAdapter.doesIlmPolicyExist('foo');
     expect(clusterClient.callAsInternalUser).toHaveBeenCalledWith('transport.request', {
       method: 'GET',
-      path: '_ilm/policy/foo',
+      path: '/_ilm/policy/foo',
     });
   });
 
@@ -78,7 +78,7 @@ describe('createIlmPolicy', () => {
     await clusterClientAdapter.createIlmPolicy('foo', { args: true });
     expect(clusterClient.callAsInternalUser).toHaveBeenCalledWith('transport.request', {
       method: 'PUT',
-      path: '_ilm/policy/foo',
+      path: '/_ilm/policy/foo',
       body: { args: true },
     });
   });

x-pack/plugins/event_log/server/es/cluster_client_adapter.ts

@@ -41,7 +41,7 @@ export class ClusterClientAdapter {
   public async doesIlmPolicyExist(policyName: string): Promise<boolean> {
     const request = {
       method: 'GET',
-      path: `_ilm/policy/${policyName}`,
+      path: `/_ilm/policy/${policyName}`,
     };
     try {
       await this.callEs('transport.request', request);
@@ -55,7 +55,7 @@ export class ClusterClientAdapter {
   public async createIlmPolicy(policyName: string, policy: unknown): Promise<void> {
     const request = {
       method: 'PUT',
-      path: `_ilm/policy/${policyName}`,
+      path: `/_ilm/policy/${policyName}`,
       body: policy,
     };
     try {