feat(proxy chain): Chained proxy support in aiohttp session

Add ChainProxyConnector support, !pin pydantic to "1.4", add
documentation on aiohttp connector.

docs/api/client/session/aiohttp.md

@@ -0,0 +1,76 @@
+# Aiohttp session
+
+AiohttpSession represents a wrapper-class around `ClientSession` from [aiohttp]('https://pypi.org/project/aiohttp/')
+
+Currently `AiohttpSession` is a default session used in `aiogram.Bot`
+
+## Usage example
+
+```python
+from aiogram import Bot
+from aiogram.api.client.session.aiohttp import AiohttpSession
+
+session = AiohttpSession()
+Bot('token', session=session)
+```
+
+
+## Proxy requests in AiohttpSession
+
+In order to use AiohttpSession with proxy connector you have to install [aiohttp-socks]('https://pypi.org/project/aiohttp-socks/')
+
+Binding session to bot:
+```python
+from aiogram import Bot
+from aiogram.api.client.session.aiohttp import AiohttpSession
+
+session = AiohttpSession(proxy="protocol://host:port/")
+Bot(token="bot token", session=session)
+```
+
+!!! note "Protocols"
+    Only following protocols are supported: http(tunneling), socks4(a), socks5 as aiohttp_socks documentation claims.
+
+
+### Authorization
+
+Proxy authorization credentials can be specified in proxy URL or come as an instance of `aiohttp.BasicAuth` containing 
+login and password.
+
+Consider examples:
+```python
+from aiohttp import BasicAuth
+from aiogram.api.client.session.aiohttp import AiohttpSession
+
+auth = BasicAuth(login="user", password="password")
+session = AiohttpSession(proxy=("protocol://host:port", auth))
+# or simply include your basic auth credential in URL
+session = AiohttpSession(proxy="protocol://user:password@host:port")
+```
+
+!!! note "Credential priorities"
+    Aiogram prefers `BasicAuth` over username and password in URL, so 
+    if proxy URL contains login and password and `BasicAuth` object is passed at the same time
+    aiogram will use login and password from `BasicAuth` instance.
+
+
+### Proxy chains
+
+Since [aiohttp-socks]('https://pypi.org/project/aiohttp-socks/') supports proxy chains, you're able to use them in aiogram 
+
+Example of chain proxies:
+```python
+from aiohttp import BasicAuth
+from aiogram.api.client.session.aiohttp import AiohttpSession
+
+auth = BasicAuth(login="user", password="password")
+session = AiohttpSession(
+    proxy={"protocol0://host0:port0",
+           "protocol1://user:password@host1:port1",
+           ("protocol2://host2:port2", auth),}  # can be any iterable if not set
+)
+```
+
+## Location
+
+- `from aiogram.api.client.session.aiohttp import AiohttpSession`