Add custom header parsing to support vendor-agnostic retry

[panxjina]

Description

What?

• Add custom header parsing to support vendor-agnostic retry.
• As per current support, RetryAfterHeaderStrategy supports fixed headers for rate-limit parsing
  • RetryAfter: For throlled requests
  • X-RateLimit-Remaining/X-RateLimit-Reset: legacy headers for rate limit parsing

Why?

• In Gitlab product integration, we're encountering non-legacy ratelimit headers. (i.e. ratelimit-reset, ratelimit-remaining).
• Based on vendor support, we might have scenario to include custom rate-limit header parsing

How?

• This change includes custom header support which can be used via parameterized constructor. Added support for accepting custom rate-limit reset and remaining headers.
• This change is backward compatible, as for existing plugins using default constructor, standard legacy rate-limit headers would be supported.
• It is observed that as RFC headers will need case-insensitive match, which is already taken care by HttpHeaders methods (i.e., containsKey, getFirst).

References:-

• https://docs.gitlab.com/administration/settings/user_and_ip_rate_limits/#headers-returned-for-all-requests
• https://www.ietf.org/archive/id/draft-polli-ratelimit-headers-02.html

Issues Resolved

N/A

Check List

• New functionality includes testing.
• New functionality has a documentation issue. Please link to it in this PR.
  • New functionality has javadoc added
• Commits are signed with a real name per the DCO

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