Allow customization of Span Start Options and whole Span creation.

[aldas]

alternative solution to #19

[vishr]

@aldas the repository is now labstack/echo-otel, and main was released as github.com/labstack/echo-otel/v5 v5.0.0 (#21). Echo v4 support is on the v4 branch (github.com/labstack/echo-otel/v4 v4.0.0).

This PR needs a rebase onto the new main: the module path changed, and otel.go changed around span start and the end-of-request handling. Config.SpanStartOptions already exists; the "whole span creation" hook would be the new part. If it should also go into /v4, a follow-up PR against v4 would do it.