Add TLS support to http server

[danfaizer]

This PR provides native TLS support. Related issue: #297

In order to enable TLS you must configure:

vouch:
  tls:
    cert: /path/to/signed_cert_plus_intermediates
    key: /path/to/private_key
    # profile: Optional. Define TLS configuration:  modern, intermediate, old, default
    profile: intermediate

[bnfinet]

althought logger.Fatal is an exit event, could you please put the following srv.ListenAndServe (non TLS) in an else block for clarity

[danfaizer]

✅

[bnfinet]

Please include the the env var names in the documentation

[danfaizer]

✅

[danfaizer]

Including coverage:

./do.sh coverage
?   	github.com/danfaizer/vouch-proxy	[no test files]
ok  	github.com/danfaizer/vouch-proxy/handlers	5.728s	coverage: 71.1% of statements
?   	github.com/danfaizer/vouch-proxy/pkg/capturewriter	[no test files]
ok  	github.com/danfaizer/vouch-proxy/pkg/cfg	0.241s	coverage: 59.5% of statements
ok  	github.com/danfaizer/vouch-proxy/pkg/cookie	(cached)	coverage: 42.3% of statements
ok  	github.com/danfaizer/vouch-proxy/pkg/domains	(cached)	coverage: 79.2% of statements
?   	github.com/danfaizer/vouch-proxy/pkg/healthcheck	[no test files]
ok  	github.com/danfaizer/vouch-proxy/pkg/jwtmanager	0.296s	coverage: 46.1% of statements
?   	github.com/danfaizer/vouch-proxy/pkg/providers/adfs	[no test files]
?   	github.com/danfaizer/vouch-proxy/pkg/providers/azure	[no test files]
?   	github.com/danfaizer/vouch-proxy/pkg/providers/common	[no test files]
ok  	github.com/danfaizer/vouch-proxy/pkg/providers/github	0.478s	coverage: 72.3% of statements
?   	github.com/danfaizer/vouch-proxy/pkg/providers/google	[no test files]
?   	github.com/danfaizer/vouch-proxy/pkg/providers/homeassistant	[no test files]
?   	github.com/danfaizer/vouch-proxy/pkg/providers/indieauth	[no test files]
?   	github.com/danfaizer/vouch-proxy/pkg/providers/nextcloud	[no test files]
?   	github.com/danfaizer/vouch-proxy/pkg/providers/openid	[no test files]
?   	github.com/danfaizer/vouch-proxy/pkg/providers/openstax	[no test files]
?   	github.com/danfaizer/vouch-proxy/pkg/responses	[no test files]
?   	github.com/danfaizer/vouch-proxy/pkg/structs	[no test files]
?   	github.com/danfaizer/vouch-proxy/pkg/timelog	[no test files]