feat: rover subgraph introspect

[lrlna]

Description

Adds rover subgraph introspect command that could be run as is, or with auth headers via the --headers or -H flags. It returns sdl.

Best way to run this is:

cargo run subgraph introspect http://url-to-graphql-service.com

The who subcommand looks like so:

rover-subgraph-introspect 0.0.4
Introspect a subgraph from the Apollo registry

USAGE:
    rover subgraph introspect [OPTIONS] <endpoint>

FLAGS:
    -h, --help       Prints help information
    -V, --version    Prints version information

OPTIONS:
    -H, --header <key:value>...     headers to pass to the endpoint. Values must be key:value pairs. If a value has a
                                    space in it, use quotes around the pair, ex. -H "Auth:some key"
    -l, --log <log-level>            [possible values: error, warn, info, debug,
                                    trace]
        --profile <profile-name>    Name of configuration profile to use [default: default]

ARGS:
    <endpoint>    The endpoint of the subgraph to introspect

closes #349

[EverlastingBugstopper]

lgtm!

[EverlastingBugstopper]

why the extra newline?

[lrlna]

For readability -- I couldn't visually distinguish where introspection response started. Happy to remove tho!