Investigate if standard icons are available for immediate use in the Toolkit

[awschristou]

• Are there icons that we have access (eg a common source) to that we are allowed to use in the toolkit
• produce a comprehensive list of icons (services and commands) to request from UX

[awschristou]

We currently do not have a central collection of icons we can draw from in an api fashion. When other open source repos (with compatible licensing models, such as MIT) have svg assets that would provide a consistent UI experience in the Toolkit, we plan to copy them into this repo using the following approach:

• A separate location in this repo is used for every repo where images are sourced from. The location is in the form third-party/resources/from-<BRIEF_REPO_NAME>
• Copy the source repo's licence into this destination location's LICENSE.txt file
• Create a README.md in the destination location, and type in a copyright attribution:

The AWS Toolkit for VS Code includes the following third-party software/licensing:

Icons contained in this folder and subfolders are from <SOURCE_REPO_NAME>: <SOURCE_REPO_URL>

<PASTE_SOURCE_LICENSE_HERE>

• Copy the SVG file(s) into a suitable place within the destination location, for example .../dark/xyz.svg and .../light/xyz.svg
• Add an entry to third-party/README.md summarizing the new destination location, where the asserts were sourced from, and a brief rationale.

PR #227 illustrates what this looks like in practice.