Fix ISearchableTree in v8

[Shazwazza]

Ensure ISearchableTree is working and used in the back office

(will be part of PR #3852)

child of #3530

[Shazwazza]

Confirmed this now works. An example of how this would work (this is prior to the DI branch being merged in, so this syntax may change)

In a Compose method of an IUmbracoComponent you could do this to replace the content search:

var searchableTreeCollectionBuilder = composition.Container.GetInstance<SearchableTreeCollectionBuilder>();
searchableTreeCollectionBuilder.Exclude<ContentTreeController>();
searchableTreeCollectionBuilder.Add<MySearchableTree>();

and then include your own class, for example:

        private class MySearchableTree : ISearchableTree
        {
            public string TreeAlias => Constants.Trees.Content;

            public IEnumerable<SearchResultEntity> Search(string query, int pageSize, long pageIndex, out long totalFound, string searchFrom = null)
            {
                totalFound = 1;
                return new[]
                {
                    new SearchResultEntity
                    {
                        Id = 123,
                        Name = "Hello world",
                        Key = Guid.NewGuid(),
                        Score = 1,
                    }
                };
            }
        }

[Shazwazza]

See #3534 for other testing notes