Refactor OAuth grant infrastructure into pkg/oauthproto

[jhrozek]

Description

Prerequisite refactoring for XAA / ID-JAG support (#5218). Consolidates OAuth 2.0 grant machinery into a single home so the upcoming RFC 7523 JWT Bearer grant and RFC 8693 token exchange sit alongside shared primitives rather than being scattered across packages.

Two changes bundled as one logical unit:

• Use shared pkg/oauthproto helpers in tokenexchange — switches tokenexchange off local HTTP client and redaction constants onto the shared oauthproto.DefaultHTTPClient() and oauthproto.Redact() helpers. Every grant now uses the same process-wide transport and connection pool.
• Move tokenexchange under pkg/oauthproto — relocates pkg/auth/tokenexchange → pkg/oauthproto/tokenexchange. Pure rename; all import paths updated. With this move pkg/oauthproto becomes the single home for all OAuth 2.0 grant machinery.

Closed by

• Use shared pkg/oauthproto helpers in tokenexchange #5212 (Use shared pkg/oauthproto helpers in tokenexchange)
• Move tokenexchange under pkg/oauthproto #5251 (Move tokenexchange under pkg/oauthproto)

[jhrozek]

Closed by #5212 and #5251.