Implement nested form binding for structs and arrays

[kankankanp]

Summary

Addresses issue #2746 by adding support for binding nested structs, arrays, and pointer fields from form data in Echo.
This enhancement allows developers to seamlessly bind complex, deeply nested web form data (such as team.members[0].name=Alice) directly to Go structs with nested slices, pointer fields, and multiple levels of depth.

Changes

• Implements recursive parsing and binding for form keys using dot notation and array indices (e.g. team.members[0].name).
• Supports deeply nested structs, slices, and pointer fields within form binding.
• Updates binder logic to handle keys like winner.players[1].role=Forward and loser.leaders[0].name=Charlie.
• Adds comprehensive tests for nested, pointer, sparse, and edge-case bindings.

Benefits

• Enables direct binding of complex HTML form data to Go structs, reducing manual mapping in user applications.
• Addresses feature requests such as #2746.
• Maintains backward compatibility with existing flat struct form bindings.
• Improves ergonomics for web forms with nested data in Echo.

Test plan

• All new and existing tests pass
• Linting passes
• Manual verification with sample Echo server and curl requests
• No behavioral changes for existing flat form bindings

[codecov]

Codecov Report

❌ Patch coverage is 81.60920% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.18%. Comparing base (f24aaff) to head (1555007).
⚠️ Report is 17 commits behind head on master.

Files with missing lines	Patch %	Lines
bind.go	81.60%	11 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2834      +/-   ##
==========================================
- Coverage   93.25%   93.18%   -0.07%     
==========================================
  Files          39       39              
  Lines        4652     4753     +101     
==========================================
+ Hits         4338     4429      +91     
- Misses        218      225       +7     
- Partials       96       99       +3     

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