golang/oauth2: added OnTokenRefresh callback method

[Dainismo]

This pr implements the most simple way of attaching a callback for token refreshes. Whenever the token gets refreshed by the client, this method will be called. This allows to get notified whenever the access token or refresh token expires and be able to store those new credentials inside a database for future requests.

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Patch Set 1:

(1 comment)

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Patch Set 1:

(2 comments)

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Patch Set 1:

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Patch Set 3:

(1 comment)

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Patch Set 4:

(1 comment)

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Patch Set 5:

(1 comment)

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Patch Set 5: Code-Review+1

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