OAuth2 KeyFileProvider - Use the DATA constant as prefix

[Niennienzz]

Motivation

• Code cleanup.

Modifications

• A minor change to use the constant instead of a repeated string.

Verifying this change

• Make sure that the change passes the CI checks.
• This change is already covered by existing tests: TestNewAuthenticationOAuth2WithParams in pulsar/internal/auth/oauth2_test.go.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API: (yes / no)
• The schema: (yes / no / don't know)
• The default values of configurations: (yes / no)
• The wire protocol: (yes / no)

Documentation

• Does this pull request introduce a new feature? - No.

[tisonkun]

@nodece shall we merge this patch now or close it when we're merging #883?

[tisonkun]

Merging...