Apigee - Add forward_proxy_uri support on environment resource

[diogo-j-n-teixeira]

Problem:

Apigee now support the use of an Forward Proxy. That configuration is at environment level, and the Terraform resource already have the ability of use it. However, this Apigee module don't.

Solution:

Add the argument forward_proxy_uri. This was tested, and the behavior is:

• Having null"" or no definition of the forward_proxy_uri argument is the same for the provider, so no change/drift is raised
• If you pass {scheme}://{hostname}:{port} TF will update the resource

Documentation:

• Use of forward proxy - Here
• Rest API - Here
• Terraform resource - Here

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Checklist

I applicable, I acknowledge that I have:

• Read the contributing guide
• Ran terraform fmt on all modified files
• Regenerated the relevant README.md files using tools/tfdoc.py
• Made sure all relevant tests pass

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