oauth: mark NewOauthAccess as deprecated and update examples to use TokenSource

[buzzsurfr]

Fixes #5415

SUMMARY OF CHANGES

• Mark oauth.NewOauthAccess as deprecated, pointing to oauth.TokenSource
• Change authentication and interceptor example clients to use oauth.TokenSource
• Change interop to use oauth.TokenSource

RELEASE NOTES:

• Mark oauth.NewOauthAccess as deprecated

[buzzsurfr]

FYI, go's parser gives a warning one of the files in interop, stating:

"io/ioutil" has been deprecated since Go 1.16: As of Go 1.16, the same functionality is now provided by package io or package os, and those implementations should be preferred in new code. See the specific function documentation for details.  (SA1019)

The offending line is interop/client/client.go#L156

b, err := ioutil.ReadFile(*caFile)

According to the message, this should be replaced with os.ReadFile.

Should I file a new issue and fix it with a new PR, or loop it into this one?

UPDATE: I found 42 references to ioutil.ReadFile in the repo, so would also need to know whether to fix one or all.

[easwars]

Should I file a new issue and fix it with a new PR, or loop it into this one?

UPDATE: I found 42 references to ioutil.ReadFile in the repo, so would also need to know whether to fix one or all.

Given that we have so many references, a separate issue would be more appropriate. Thanks.

[arvindbr8]

LGTM

[easwars]

@buzzsurfr : Thank you for your contribution !!