Add HTTP client context message to TokenSource

[LeviHarrison]

While the other two methods of obtaining a token in the clientcredentials package, Client and Token, have the following message:

The provided context optionally controls which HTTP client is returned. See the oauth2.HTTPClient variable.

the third, TokenSource, does not, implying that the context can not control the HTTP client that is returned. This isn't true though, as Token uses TokenSource (https://cs.opensource.google/go/x/oauth2/+/2bc19b11:clientcredentials/clientcredentials.go;l=55-58;drc=80673b4a4bfc6c2c58a0b44cf9106913fe293994). So, we should have the same message for TokenSource.

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Message from Levi Harrison:

Patch Set 2:

(2 comments)

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Patch Set 2:

(1 comment)

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