Add kaya-session package for server-side HTTP session management

example/session.py

@@ -0,0 +1,23 @@
+from kaya.core import HttpContext, KayaApp
+from kaya.session import InMemorySessionStore, SessionMiddleware
+
+app = SessionMiddleware(KayaApp(), InMemorySessionStore())
+
+
+@app.GET('/')
+async def home(ctx: HttpContext) -> None:
+    visits = ctx.session.get('visits', 0) + 1
+    ctx.session['visits'] = visits
+    await ctx.send_str(200, f'visits: {visits}')
+
+
+@app.GET('/read')
+async def read(ctx: HttpContext) -> None:
+    visits = ctx.session.get('visits', 0)
+    await ctx.send_str(200, f'visits: {visits}')
+
+
+@app.GET('/clear')
+async def clear(ctx: HttpContext) -> None:
+    ctx.session.invalidate()
+    await ctx.send_str(200, 'session cleared')