Add credentials for bearer token authentication

[richirisu]

I recently needed bearer authentication for a proxy, however, the current HTTP implementation only offered credentials for basic and digest authentication.

It applies to the Authorization and Proxy-Authorization headers.

https://developer.mozilla.org/en-US/docs/Web/HTTP/Authentication#authentication_schemes

Implementation Details

The token provided to the HttpClientBearerCredentials is checked for valid characters according to RFC 6750.

It is possible to provide any string as token by applying base64Encode in conjunction with utf8.encode first. I was considering making a factory method that automates this, but then I felt it would just complicate things, as bearer tokens are most often randomly generated anyway.

Tests

Sorry, but I didn't provide any tests here.

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[kevmoo]

@brianquinlan – thoughts?

[mraleph]

@richirisu We can't look at the code before CLA is signed - could you sign it?

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[mit-mit]

CLA was signed; https://dart-review.googlesource.com/c/sdk/+/408241 has been assigned to Brian

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[copybara-service]

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