Implement the new klog and logfmt structured log parser

[kyasbal]

This is a change part of #357.
This PR introduces new string formatted structured logging parsers.

The previous implementations were renamed to legacyklog and it had problems:

• The implementation didn't return any error when it fails to parse field thus it couldn't identify if that was error of parsing or just receiving empty string
• The implementation used string and + operator to concatenate them. This is inefficient.
• The implementation was named like KLogParser but it actually pases KLog format or Logfmt format.

The new implementation introduced in this PR uses strings.Builder to parse and uses sync.Pool to use them efficiently.

[kyasbal]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces new, more performant parsers for klog and logfmt structured logs, moving the old implementation to a legacyklog file. The new parsers leverage sync.Pool and strings.Builder for efficiency. The changes are well-structured, with common types extracted into a new type.go file and comprehensive tests including benchmarks.

My review includes a few suggestions to improve maintainability and robustness:

• Decoupling the legacyklog parser from the logfmt parser.
• Ensuring consistent and safe usage of sync.Pool.
• Improving test file organization.