[dart][dart-dio-next] Authentication fixes and improvements

[kuhnroyal]

Fixes #9906

• update http mock test library
• correctly map authentication by type and scheme
• add new authentication interceptor for bearer scheme - this currently does the same as the OAuth interceptor
• add tests for all authentication types except OAuth
• use temporary test library branch until new fixes and features get merge there

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CC @swipesight (2018/09) @jaumard (2018/09) @josh-burton (2019/12) @amondnet (2019/12) @sbu-WBT (2020/12) @kuhnroyal (2020/12) @agilob (2020/12)

[kuhnroyal]

This is ready for reviews. Just waiting for an updated release of the mock library but other than that I expect no more changes.

[jaumard]

Looks good to me :)

[agilob]

Looks good

[kuhnroyal]

Updated the mock library after its release. Should be good to merge once it builds.