Move DefaultFunc logic to configure section

[megan07]

Background: Running the configured provider when access_token is a static value and credentials are set by an env variable fails, it seems the test only succeeded previously because access_token was set to an unknown resource attribute. And it fails with 0.15 because there is a new change that validates the complete configuration before configuring the provider.

In this case it seems like an explicitly-set access_token should override a defaulted credentials, but we aren't sure that assumption should be generalized for all ConflictsWith/DefaultFunc situations. So the suggestion was that maybe we leave DefaultFunc unset and handle the environment variables directly inside Configure instead.

We'll need to review the precedence of access_token and credentials after the logic is moved into Configure. This will be a breaking change so we will address it in 4.0.0

Failed test: https://ci-oss.hashicorp.engineering/buildConfiguration/GoogleCloud_ProviderGoogleCloudGoogleProject/180134?showLog=180134_209134_208405.208416.209134&logFilter=debug

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