Replace enums HttpMethod and HttpStatus with String/int or provide access to "raw" values

[thc202]

By spec an HTTP method can be anything (within the token format), use of enum for the method leads to failure to read requests that use a method that's not currently supported by the library (e.g. #13).
Similarly to the status codes (e.g. #20). For correctness, it's possible to read HAR with status code not supported by the library but it ends up with 0 status code (the UNKNOWN_HTTP_STATUS).

https://httpwg.org/specs/rfc9110.html#rfc.section.9.1
https://httpwg.org/specs/rfc9110.html#status.codes

For registered methods: https://www.iana.org/assignments/http-methods/http-methods.xhtml
For registered status codes: https://www.iana.org/assignments/http-status-codes/http-status-codes.xhtml
(As example of methods/status codes that are officially registered but not supported by the library.)

Happy to pull request a solution.