External client configuration sample

[THardy98]

What was changed

Basic samples for environment configuration, showing default usage and usage with a specific profile and environment variables overrides.

Partly addresses #178

2. How was this tested:
   Samples run correctly

3. Any docs updates needed?
   I don't think so

[THardy98]

Would like to wait for temporalio/sdk-core#968 before changing the existing samples, so we can simplify the diff (otherwise we'll need to read & load from a config file)

[cretz]

I assume the updating of all existing samples to leverage the one-liner for loading from env config will be a separate PR?

[THardy98]

Yes - I am updating samples to have their own TOML config (or shared) and loading from there.

Would need to release new version of Python SDK with temporalio/sdk-python#1004 included, to avoid samples needing to have TOML config for the default profile.

But I'd rather update these samples than wait for another release. Once the new release is out, we can update it accordingly (would be a small diff).

[THardy98]

#233