Add new ResolverWithOpts

[jdolitsky]

This will allow us to add new options to Resolver without breaking API in the future.

In the near term, this allows us to set a custom User-Agent header:

headers := http.Header{}
headers.Set( "User-Agent", "my-user-agent")
resolver := ResolverWithOpts(WithResolverHeaders(headers))

Resolves oras-project/oras#240

[deitch]

A few things I noticed.

• We used to pass a context.Context to Resolver(). Do we not need it anymore?
• In other areas, functions to set options passed as variadic parameters always were named With*, which would make these WithResolverOptClient(client *http.Client) or WithResolverOptPlainHTTP(). Should we be consistent in the naming?
• whether or not to use plain http is a boolean, which defaults to false. If the only thing we will do is set it to true, then it probably should just be ResolverOptPlainHTTP() (or WithResolverOptPlainHTTP(), not parameter required

Right now, just a comment.

[jdolitsky]

@deitch -

We used to pass a context.Context to Resolver(). Do we not need it anymore?

It's actually not needed. The original implementation was just throwing it away (Resolver(_ context.Context, ...). The beauty of this is that if it is needed later, we can add a new WithResolverContext(context.Context) option.

In other areas, functions to set options passed as variadic parameters always were named With*, which would make these WithResolverOptClient(client *http.Client) or WithResolverOptPlainHTTP(). Should we be consistent in the naming?

Yes, agree. Done.

whether or not to use plain http is a boolean, which defaults to false. If the only thing we will do is set it to true, then it probably should just be ResolverOptPlainHTTP() (or WithResolverOptPlainHTTP(), not parameter required

Done.