Add `compare_content` function to `Enr` to allow comparisons modulo signature #171

Job	Run time
cargo-fmt	9s
Check rustdoc intra-doc links	31s
cargo-clippy	27s
cargo-audit	5m 45s
release-tests-ubuntu	35s
release-tests-ubuntu-ed25519	42s
release-tests-ubuntu-all-features	37s
	8m 46s

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