Use go-plugin's internal mechanism for verifying checksum

[mostafa]

This requires checksums to be loaded securely, rather than being hard-coded in the plugin config file.

[mostafa]

This should be done at a later time and I'll remove it from the v0.2.0 milestone. The reason is that there should be a trusted repository to hold the checksums, so that GatewayD can verify the plugins against them. Otherwise the plugins can be downloaded from GitHub and the checksums should reside next to the release files and used in the gatewayd_plugin.yaml file.

[mostafa]

The gatewayd-plugin-cache is released. Each gunzipped tar file contain the binary and the checksum of the binary file. The plugin command (#122) can leverage this to include the checksum in the gatewayd_plugins.yaml config. The checksum validation of GatewayD should be removed in favor of go-plugin's.