[FEATURE] Allow opting out from validation of header values

Tapir version: 1.0.0

Scala version: 2.13.8

Description
original post in gitter: https://gitter.im/softwaremill/tapir?at=62b1f6000a522647985b7307
Consider an endpoint which define a response header based on some configuration:

.out(header("Keep-Alive", s"timeout=$keepAliveSeconds"))

same endpoint is reused in an sttp client.
Now client does not necessarily have same configuration as server, thus that endpoint looks a bit different.
When trying to use the sttp client, one would get something of this sort:

Mismatch(Some(Keep-Alive: timeout=0),Some(Keep-Alive: timeout=3600))

But this is not very helpful.
A possible solution might have an attribute we can set on the input, which would be used when decoding.

Start by tracing how header inputs are validated during decoding and how mismatches are reported for reused endpoints. Define an opt-out mechanism that permits differing configured header values between the server and sttp client, then add coverage for the Keep-Alive example and verify that validation remains unchanged when opting out is not requested.