src/erasure-code: support statically-linked (built-in) plugins

src/erasure-code: support statically-linked (built-in) plugins

This commit adds the functionality in EC plugin registry to support
built in (statically linked) plugins. This is done by 2 main changes:

1. introducing a new map, builtin_plugins, where plugins can register
a factory for itself at startup.
2. load() now first checks if the plugin is builtin and instantiates it,
otherwise it falls back to dynamically linking the plugin.

Signed-off-by: Shraddha Agrawal <shraddha.agrawal000@gmail.com>