Implement the runtime metadata file (toml)

[Angelmmiguel]

These kind of files will be managed by wws, so it requires a representation in the source code. For consistency with the project, we chose the format as TOML.

The purpose of them is to "teach" wws how to use certain runtime and which files can be run with it. For this reason, it must include information about the name, version and binary, but also polyfills, arguments and environment variables.

Content

Property	Type	Description
spec_version	u32	The version of this file
name	String	The runtime name
version	String	The runtime version number
binary	String	URL to download the binary
binary_checksum	String	Binary checksum
extensions	String[]	Associated file extensions to this runtime
polyfill (optional)	String	An associated source file with polyfills. It will be mounted inside lib folder using WASI
polyfill_checksum (optional)	String	The checksum to validate the polyfill file
template (optional)	String	An optional template in case the source code needs to be modified. For example, to load a library or to add a function call. The provided code will be available in the {code} key
template_checksum (optional)	String	The checksum to validate the template file
args	String[]	Arguments to pass in the WASI context
envs (optional)	HashMap<String, String>	Default environment variables
folders (optional)	HashMap<String, Object>	A list of folders to mount with this runtime. It includes URL to download a compressed file, mount point and if decompression is required.

We may change these names or add some of them in the future.

It's related to #63

[Angelmmiguel]

This is duplicated from #65