Add header flag parser and passthru for introspection

[JakeDawkins]

These changes are fairly small, but add the ability to pass through headers, similarly to how curl works with --header or -H.

Valid examples:

# single headers
rover graph introspect http://localhost:4000 --header auth:hello
rover graph introspect http://localhost:4000 -H auth:hello
rover graph introspect http://localhost:4000 -H "auth:hello"
rover graph introspect http://localhost:4000 --header auth:"hello"
rover graph introspect http://localhost:4000 --header "auth:value with spaces"
rover graph introspect http://localhost:4000 --header auth:"value with spaces"

# I think reqwest is stripping leading whitespace from header values automatically (which is what we'd want), 
# even though with the current implementation, leading whitespaces are added to the header map
rover graph introspect http://localhost:4000 --header "auth: value with leading spaces"

# multiple headers
rover graph introspect http://localhost:4000 --header auth:hello another:header
rover graph introspect http://localhost:4000 --header auth:hello --header another:header
rover graph introspect http://localhost:4000 --header auth:hello -H another:header
rover graph introspect http://localhost:4000 --header auth:hello -H "another-header:with spaces"

[lrlna]

I think if it has a :, it'll likely be in a quoted string. The only : that's likely to be there is the one that separates the header key and the header value. The values are just going to be comma separated. That is to say the parsing you have below seems correct to me.

From the spec:

message-header = field-name ":" [ field-value ]
field-name     = token
field-value    = *( field-content | LWS )
field-content  = <the OCTETs making up the field-value
                 and consisting of either *TEXT or combinations
                 of token, separators, and quoted-string>