Python: Consider ways for developers to recover more gracefully when running examples with incomplete .env

[pamelafox]

For the examples, they rely currently on everything being in .env, which is great when your .env is setup correctly, but is confusing when its not, since the error messages talk about constructor parameter names.
Some ideas:

1. Including env variable names in error messages as well ("this parameter can also be set via the en variable bla")
2. Always explicitly passing in every necessary parameter name, like deployment_name=os.get_env("AZURE_CHAT_DEPLOYMENT_NAME")
3. at least put the required env variable names in each example above the Client()
4. Make a .env.sample with everything that might be needed for all the samples

My preference is #2!

[pamelafox]

With #2, then I prefer adding:

load_env(override=True)

top the top of the file, using python-dotenv package import.

[dmytrostruk]

4. Make a .env.sample with everything that might be needed for all the samples

Added: https://github.com/microsoft/agent-framework/blob/main/python/.env.example

[dmytrostruk]

1. Including env variable names in error messages as well ("this parameter can also be set via the en variable bla")

Added in this PR: #539

[dmytrostruk]

2. Always explicitly passing in every necessary parameter name, like deployment_name=os.get_env("AZURE_CHAT_DEPLOYMENT_NAME")

Added examples in this PR: #562