Feature: AgentExecutor execution time limit (#2399)

`AgentExecutor` already has support for limiting the number of
iterations. But the amount of time taken for each iteration can vary
quite a bit, so it is difficult to place limits on the execution time.
This PR adds a new field `max_execution_time` to the `AgentExecutor`
model. When called asynchronously, the agent loop is wrapped in an
`asyncio.timeout()` context which triggers the early stopping response
if the time limit is reached. When called synchronously, the agent loop
checks for both the max_iteration limit and the time limit after each
iteration.

When used asynchronously `max_execution_time` gives really tight control
over the max time for an execution chain. When used synchronously, the
chain can unfortunately exceed max_execution_time, but it still gives
more control than trying to estimate the number of max_iterations needed
to cap the execution time.

---------

Co-authored-by: Zachary Jones <zjones@zetaglobal.com>

pyproject.toml

@@ -57,6 +57,7 @@ pgvector = {version = "^0.1.6", optional = true}
 psycopg2-binary = {version = "^2.9.5", optional = true}
 boto3 = {version = "^1.26.96", optional = true}
 pyowm = {version = "^3.3.0", optional = true}
+async-timeout = {version = "^4.0.0", python = "<3.11"}
 
 [tool.poetry.group.docs.dependencies]
 autodoc_pydantic = "^1.8.0"