Add build.edge_handlers configuration property

[ehmicky]

Based on https://github.com/netlify/pod-the-builder/issues/51

We should add a build.edge_handlers configuration property.

[build]
edge_handlers = "src/my-edge-handlers"

This configuration property should default to ./edge-handlers. Paths are handled like other file paths in netlify.toml, e.g. relative paths are relative to the build directory.

The file path, after being normalized like the build.functions path is, should be passed to plugins as constants.EDGE_HANDLERS_SRC.

[ehmicky]

Done in #1828 and #1829.