[typescript-angular] Unify authorization lookup

[djnalluri]

Fixes #6944

A new credentials property is added to the configuration object. The property maps authentication scheme names to the values that go in the 'Authorization' header. The values are represented as either a string or a function that returns one. The property is filled with default functions for all known authentication schemes that execute fallback behaviour. API calls use the new property for all lookups.

PR checklist

• Read the contribution guidelines.
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• Run the shell script ./bin/generate-samples.shto update all Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master. These must match the expectations made by your contribution. You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/config/java*. For Windows users, please run the script in Git BASH.
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Technical Committee:
@TiFu (2017/07) @taxpon (2017/07) @sebastianhaas (2017/07) @kenisteward (2017/07) @Vrolijkx (2017/09) @macjohnny (2018/01) @topce (2018/10) @akehir (2019/07) @petejohansonxo (2019/11) @amakhrov (2020/02)

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[macjohnny]

please add some docs, but otherwise it looks good to me