This PR adds a new `--beacon.checkpoint.file` config flag to geth and
blsync which specifies a checkpoint import/export file. If a file with
an existing checkpoint is specified, it is used for initialization
instead of the hardcoded one (except when `--beacon.checkpoint` is also
specified simultaneously). Whenever the client encounters a new valid
finality update with a suitable finalized beacon block root at an epoch
boundary, it saves the block root in hex format to the checkpoint file.
This PR moves chain config related code (config file processing, fork
logic, network defaults) from `beacon/types` and `beacon/blsync` into
`beacon/params` while the command line flag logic of the chain config is
moved into `cmd/utils`, thereby removing the cli dependencies from
package `beacon` and its sub-packages.
