oauth2: support client assertion in client_credentials

[zetaab]

This PR will add client assertion support for client_credentials.
Like mentioned in #744 this library is missing client_assertion
support, which is kind of important feature nowadays as people does
not want to use static credentials.

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Patch Set 1:

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Message from Jesse Haka:

Patch Set 1:

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Message from Jesse Haka:

Patch Set 2:

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Message from Sean Liao:

Patch Set 5: Hold+1

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Patch Set 5:

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Patch Set 5:

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Message from Sean Liao:

Patch Set 5:

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