Fixes #226 - Does not contain a definition for AddJsonSessionKeySerializer

[br3nt]

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PR Title

Update code example in core.md to match the current API and start without errors.

PR Description

Code example snippet changed to:

builder.Services.AddSystemWebAdapters()
    .AddJsonSessionSerializer(options => ClassLibrary.SessionUtils.RegisterSessionKeys(options))
    .AddRemoteAppClient(options =>
    {
        options.RemoteAppUrl = new(builder.Configuration["ReverseProxy:Clusters:fallbackCluster:Destinations:fallbackApp:Address"]);
        options.ApiKey = builder.Configuration["RemoteAppApiKey"];
    })
    .AddSessionClient();

builder.Services.AddReverseProxy().LoadFromConfig(builder.Configuration.GetSection("ReverseProxy"));

from:

builder.Services.AddSystemWebAdapters()
    .AddJsonSessionKeySerializer(options => ClassLibrary.SessionUtils.RegisterSessionKeys(options))
    .AddRemoteAppClient(options =>
    {
        options.RemoteAppUrl = new(builder.Configuration["ReverseProxy:Clusters:fallbackCluster:Destinations:fallbackApp:Address"]);
        options.ApiKey = builder.Configuration("RemoteAppApiKey");
    })
    .AddSessionClient();

Addresses #226

[br3nt]

Also fixes most of #214

[adityamandaleeka]

Thanks @br3nt