fx.go

This adds a plugin interface that lets the plugin modify the fx
options that are passed to fx when the app is initialized. This means
plugins can inject their own implementations of IPFS interfaces. This
enables granular customization of go-ipfs behavior by plugins, such
as:

- Bitswap with custom filters (e.g. for CID blocking) Custom interface

- implementations such as Pinner or DAGService

- Dynamic configuration of libp2p ...

One downside of this is that we're exposing the entire dependency
graph, init hooks, initialization, etc. to users, so this comes with a
caveat that we reserve the right to make breaking changes to the graph
structure and initialization logic (although this historically happens
rarely). If these things are changed, we should mention them in
release notes and changelogs though, since they could impact users of
this plugin interface.

I'm not particularly fond of DI frameworks (and neither are some of
the folks work on/near go-ipfs), but it seems unlikely that somebody
will rewrite the dependency wiring and lifecycle hooks of go-ipfs, and
add dynamic extension points, so this seems like a palatable
compromise.

There are also problems that we should clean up in how model the
go-ipfs app in fx, such as:

- We make extensive use of nested fx.Options, which fx itself
discourages because it "limits the user's ability to customize their
application". It should be easy to flatten these out into a single
[]fx.Option slice.

- We pass around a list of opaque libp2p opts, which makes it hard to
customize after-the-fact...we should consider naming each of these
opts and providing them to fx as proper dependencies, so that they can
be explicitly overridden.

- We call fx.Invoke() in some places with anonymous functions. We
should instead only pass exported functions to fx.Invoke(), so that
they have exported names, which would make it easier to remove/augment
the invocations that happen when the app is initialized.

These aren't blocking issues, they just make it harder and more
brittle to customize go-ipfs with this plugin.