Add support for configuring a GoogleAdsClient using environment variables

[jradcliff]

Also added support for configuring linkedCustomerId from a config
file, properties, or an environment variable.

[jradcliff]

Note to reviewers:

I considered multiple approaches here, but having a private GoogleCredentials.Builder was the cleanest approach I could find that satisfied all of the following requirements:

1. Support merging values from environment variables, a config file, properties, and setter invocations.
2. Do not make any breaking changes to the public interface of the client or its builder.
3. Support either setting credentials explicitly via setCredentials(Credentials) or configuring credentials from a file, properties, or environment variables.
4. Play nice with AutoValue. An unfortunate result of this requirement is that I had to mark getCredentials() on the client as @Nullable. There were alternatives to doing so, but they were all considerably less pleasant.

[jradcliff]

Heads up @nwbirnie and @devchas : I was able to find a way to implement "last one wins" logic for the 3rd requirement:

3. Support either setting credentials explicitly via setCredentials(Credentials) or configuring credentials from a file, properties, or environment variables.

With the latest commit, users who previously took the following steps in either order will see the same behavior as always:

• Call setCredentials(Credentials).
• Call fromProperties(...).

That is, the last step they performed will configure their credentials. My prior commits on this PR would have made either sequence throw an exception, which would have been a breaking change.

[nwbirnie]

First round of comments - going to take another pass just now

[nwbirnie]

Why do we need state to represent the environment? Can't we just load the environment as required?

It looks like you could use a Supplier<Map<String,String>> to represent abstract this for testing purposes.

[nwbirnie]

This is a nit.

[jradcliff]

Good suggestion. I swapped this out with a Function<String, String> and it worked out quite nicely + avoided copying the environment map.

[nwbirnie]

LGTM pending open comments

[jradcliff]

I merged from master at d632e3ae in preparation for merging this. Please use Jump to to look at the two files that actually changed: GoogleAdsClient.java and GoogleAdsClientTest.java.