Lazy header parsing

[emiago]

To gain more performance we could implement lazy parsing within sip headers.
Currently we have some default parsing which could be reduced
https://github.com/emiago/sipgo/blob/main/parser/parse_header.go#L18

Currently lazy parsing can only be done for rest of headers as they are only stored as key:value
Some of parser optimized functions are exposed like
ParseUri ParseAddressValue
but we could expose more?
So in case where header is not preparsed like Via, To, From .. caller must do this manually
`
hdr := msg.GetHeader("XXX")
uri, _ := parser.ParseUri(hdr.Value())

Now parsing can be also moved to fetching header, where caching is done on demand. This could
increase performance, specially in case where messages get discarded.