Support data integrity header

[webb-ben]

Is your feature request related to a problem? Please describe.
To enhance data integrity verification in pygeoapi responses, we should consider adding the Content-Digest header. This would allow clients to validate that the response body has not been tampered with during transmission.

Describe the solution you'd like
I have a working implementation that takes the Want-Content-Digest and returns the corresponding digest as supported. This is included in a similar way that the apply_gzip function is implemented. I think this could similarly be configurable as gzip, however not sure what the server config would want to specify (acceptable digest methods, sending digest without it being requested).

Describe alternatives you've considered
I know this may be applied in an upstream webserver. To do so in nginx would require custom njs. Similar to gzip I still think this is a functionality that is important to be a part of pygeoapi.

Additional context
It would be interesting to think of a plugin oriented structure to apply the changes to header and content that we are making with gzip and content integrity to pygeoapi responses. Not sure what this would look like.