Add initialization hook to customize botocore sessions

[jamesls]

This adds an initialization hook to customize any botocore sessions.

Botocore sessions provide context for creating one or more clients.
It allows you to provide configuration, components, hooks, etc. that
can be shared across all clients created within that session.

There is no similar concept for sessions. Specifically, there is no way
to provide a common set of configuration, components, hooks, etc. that
can shared across all sessions created within botocore.

This PR adds this initialization hook. You can now register an initialization
callback that's invoked whenever a session is created:

import botocore

def my_initializer(session: botocore.session.Session) -> None:
    session.register_component('data_loader', MyNewCrazyLoader())

botocore.register_initializer(my_initializer)

Now every session will automatically use this new loader:

some_session = botocore.session.get_session()
assert isinstance(
    some_session.get_component('data_loader'), MyNewCrazyLoader
)

Rationale

Botocore's extensibility points allows customers to customize the behavior of
botocore. However, in order to leverage these extensibility points, customers
must write explicit code to register these hooks that requires a reference to a
specific session (e.g session.register, session.register_component,
session.set_default_client_config). There is no way to provide "plugin" like
behavior where customers can declaratively specify functionality without having
to modify all their existing code.

Here's one concrete use case. Botocore has a data loader component that's
responsible for loading the service model based on various parameters
(service_name, type_name, api_version). The current implementation uses
JSON, which is not efficient in terms of disk space nor load time. There have
been initiatives to improve this (#2628), but in order to make this default
behavior, the existing JSONFileLoader class had to be modified.
There are also crazier use case specific ideas to further optimize both the
load time and disk size of our service models. However, in order to use
a new loader, users must add this line of code to every session they create:

s = botocore.session.get_session()
new_fancy_loader = MyNewCrazyLoader()
s.register_component(new_fancy_loader)

This is fine if you are able to obtain a reference to every single
session you create, but this isn't always the case, especially if
you're not directly using a session (e.g. some high level library
that wraps sessions/clients for you).

I maintain a framework, Chalice, where I would like to allow users
to opt-in to more optimized data loaders through a framework-specific
config file, without having to change any of their existing code.

[codecov-commenter]

Codecov Report

Merging #2682 (e017467) into develop (92247f6) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #2682   +/-   ##
========================================
  Coverage    95.29%   95.30%           
========================================
  Files           60       60           
  Lines        12252    12261    +9     
========================================
+ Hits         11676    11685    +9     
  Misses         576      576           

Impacted Files	Coverage Δ
botocore/__init__.py	100.00% <100.00%> (ø)
botocore/session.py	96.79% <100.00%> (+<0.01%)	⬆️

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[dlm6693]

Looks good to me! Only thought is maybe it would be useful to have a method that invokes specific initializers rather than all of them carte blanche. Would require a refactor since invoke_initializers is called at initialization, but just an idea! 🚢

[nateprewitt]