[Fleet] Migrate server-side plugin to synchronous lifecycle

[pgayvallet]

Required for #53268

In #89562, we migrated almost all plugins to synchronous lifecycle

The server-side part of the fleet plugin was not migrated, because its start method is initializing a service asynchronously

kibana/x-pack/plugins/fleet/server/plugin.ts

Lines 282 to 284 in 3b3327d

	public async start(core: CoreStart, plugins: FleetStartDeps): Promise<FleetStartContract> {
	await appContextService.start({
	elasticsearch: core.elasticsearch,

AppContextService.start is only asynchronous because it needs to wait for the initial config emission:

kibana/x-pack/plugins/fleet/server/services/app_context.ts

Lines 56 to 61 in 4584a8b

	if (appContext.config$) {
	this.config$ = appContext.config$;
	const initialValue = await this.config$.pipe(first()).toPromise();
	this.configSubject$ = new BehaviorSubject(initialValue);
	this.config$.subscribe(this.configSubject$);
	}

Now that we introduced a synchronous API to read a plugin's config in #88981, the service could be changed to be synchronous

Note that the only reason I didn't do it in *** is that I wasn't sure if the this.config$.subscribe(this.configSubject$); line was still making any sense now that we have synchronous config access, and wanted to let the owners decide on that one. but AFAIK the changes are trivial.

@elastic/fleet ideally, this would be done for 7.13

cc @elastic/kibana-core