[feat] Support DB-less YAML files as the source of truth for configuration

[zhouzhuojie]

UI is good, but sometimes we also want to codify the configuration of flags into code, and we can have PRs and code reviews for flags.

Currently, flagr supports https://github.com/checkr/flagr/blob/master/pkg/config/env.go#L51-L59

	FLAGR_DB_DBDRIVER     FLAGR_DB_DBCONNECTIONSTR
	=================     ===============================================================
	"sqlite3"             "/tmp/file.db"
	"sqlite3"             ":memory:"
	"mysql"               "root:@tcp(127.0.0.1:18100)/flagr?parseTime=true"
	"postgres"            "postgres://user:password@host:5432/flagr?sslmode=disable"
	"json_file"           "/tmp/flags.json"                    # (it automatically sets EvalOnlyMode=true)
	"json_http"           "https://example.com/flags.json"     # (it automatically sets EvalOnlyMode=true)

We can also have yaml_file support with a well-documented schema of the yaml blocks.